Image display device

ABSTRACT

An image display device includes: a projection port that projects image display light generated based on an image signal; and a combiner that presents a virtual image by reflecting the image display light projected from the projection port. With respect to a specific direction along a reflection surface of the combiner, the curvature of the combiner in the specific direction becomes smaller as a distance from the projection port becomes larger. The reflection surface of the combiner may be formed by a biconic surface.

BACKGROUND

1. Field of the Invention

The present invention relates to an image display device andparticularly to an image display device that presents an image based onimage display light to a user as a virtual image.

2. Description of the Related Art

Image display devices called head up displays are known. Head updisplays have an optical element called combiner. This combiner allowsexternal light to pass through and reflects image display lightprojected from an optical unit provided in a head up display. Thisallows the user to visually recognize an image related to the imagedisplay light while overlapping the image on a landscape via thecombiner.

These image display devices called head up displays have receivedattention as in-vehicle image display devices in recent years since theimage display devices allow a driver of a vehicle to also recognize,almost without changing the line of sight or a focus for visuallyrecognizing a view outside the vehicle, information of an imageprojected from an optical unit.

For example, Patent document No. 1 discloses a head up display to bemounted on a dashboard of a vehicle that adjusts, by using an X-axisstage, a Z-axis stage, and a rotation stage, a space that can bevisually recognized by a user.

[Patent document No. 1] JP 10-278629

Head up displays such as those described above are required to be assmall as possible. For example, in a head up display to be mounted in avehicle, since a position and a space where the head up display can beattached are limited, a combiner to be mounted is required to be assmall as possible. On the other hand, presentation of a large virtualimage on a small combiner in order to increase visibility of a user, whois a driver, may cause distortion in an image due to increased effect ofaberration.

SUMMARY

In this background, a purpose of the present invention is to provide asmall image display device that allows for a reduction in distortion inan image caused by aberration even when a position and a space forattachment are limited.

One embodiment of the present invention relates to an image displaydevice. The device includes: an image generation unit that generatesimage display light based on an image signal; a projection port thatprojects the image display light generated by the image generation unit;and a combiner that presents a virtual image by reflecting the imagedisplay light projected from the projection port. With respect to aspecific direction along a reflection surface of the combiner, thecurvature of the combiner in the specific direction becomes smaller as adistance from the projection port becomes larger.

According to the present invention, a small image display device can beprovided that allows for a reduction in distortion in an image caused byaberration even when a position and a space for attachment are limited.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 is a perspective view of a head up display, which is an imagedisplay device for a vehicle according to an embodiment of the presentinvention, shown by means of a field of view from the inside of thevehicle;

FIG. 2 is a perspective view of the head up display in FIG. 1 shown bymeans of a field of view from the side of a windshield;

FIG. 3 is a diagram illustrating the internal configuration of anoptical unit along with an optical path;

FIG. 4 is an exploded perspective view of an image display lightgenerator according to the embodiment;

FIG. 5A is a top view of a printed circuit board, on which an imagedisplay element is attached, viewed from an incident direction of lightfrom a light source;

FIG. 5B is a lateral view of the printed circuit board on which theimage display element is attached;

FIG. 6 is a perspective view of the image display light generatoraccording to the embodiment viewed toward an emission direction of imagedisplay light;

FIG. 7 is a perspective view of the image display light generatoraccording to the embodiment viewed from an incident direction of lightfrom the light source;

FIG. 8 is a diagram for explaining adjustment of an attachment positionin the image display light generator according to the embodiment;

FIG. 9 is a diagram illustrating the internal configuration of anoptical unit along with an optical path;

FIG. 10 is a diagram illustrating a part of the inside of the opticalunit and a part of the inside of a substrate housing portion;

FIG. 11 is a diagram illustrating a state where a heat sink and aflexible cable are removed in FIG. 10;

FIG. 12 is a lateral view of the head up display attached to a rear-viewmirror;

FIG. 13 is a front view of the head up display attached to the rear-viewmirror;

FIG. 14 is a diagram illustrating a visually recognizable region of animage (virtual image) projected onto the combiner;

FIG. 15 is a diagram illustrating a visually recognizable region of animage (virtual image) projected onto the combiner;

FIG. 16 is a perspective view illustrating the shape of the combiner;

FIG. 17 is a lateral view illustrating the combiner shown in FIG. 16;

FIG. 18 is a top view illustrating the combiner shown in FIG. 16;

FIG. 19 is a lateral view illustrating an optical path of image displaylight presented to a user via the combiner;

FIG. 20 is a lateral view illustrating an optical path of image displaylight when a viewpoint of the user, who is a driver, is moved;

FIG. 21 is a top view illustrating an optical path of image displaylight presented to the user via the combiner;

FIG. 22 is a diagram illustrating a state where a projection unit and acombiner are removed in a head up display attached for a right steeringwheel vehicle;

FIG. 23 is a diagram illustrating a state when a substrate housingportion is replaced for a left steering wheel vehicle;

FIG. 24 is a diagram illustrating a head up display replaced for a leftsteering wheel vehicle;

FIG. 25 is a perspective view illustrating an attachment member forattaching a substrate housing portion to a rear-view mirror;

FIG. 26 is a trihedral figure of an attachment plate of the attachmentmember shown in FIG. 25;

FIG. 27 is a perspective view illustrating a head up display attached toa rear-view mirror;

FIG. 28 is a cross-sectional view of a setscrew portion when a firstattachment surface of the substrate housing portion is attached suchthat the first attachment surface is in contact with an attachmentplate;

FIG. 29 is a cross-sectional view of a setscrew portion when a secondattachment surface of the substrate housing portion is attached suchthat the second attachment surface is in contact with the attachmentplate;

FIG. 30 is a diagram illustrating an exemplary variation of theattachment plate;

FIG. 31 is a lateral view illustrating a state where the combiner isfolded by a storage hinge;

FIG. 32 is a front view illustrating a state where the combiner isfolded by the storage hinge;

FIGS. 33A-33B are cross-sectional views schematically illustrating across-sectional surface of a transmission-type intermediate image screenaccording to the embodiment;

FIG. 34 is a diagram schematically illustrating a relationship among thethickness of a diffusion layer, a half-width at half-maximum angle of atransmission light distribution angle, and the resolution of a videoimage formed on the transmission-type intermediate image screen;

FIG. 35 is a diagram illustrating, in a table format, results ofresearching influence of the thickness of the diffusion layer on theresolution of a real image formed on a surface of the transmission-typeintermediate image screen by changing the thickness of the diffusionlayer, and calculated values of resolution;

FIG. 36 is a graph illustrating a relationship between the thickness ofthe diffusion layer and the resolution of a real image formed on thesurface of the transmission-type intermediate image screen and arelationship between the thickness of the diffusion layer and thecalculated values of the resolution;

FIG. 37 is a perspective view illustrating the exterior appearance of anon-dashboard-type head up display according to the embodiment;

FIG. 38 is a diagram schematically illustrating a relationship betweenan installation position of the on-dashboard-type head up display andthe position of a virtual image presented to a driver;

FIG. 39 is a cross-sectional view schematically illustrating across-sectional surface of a reflection-type intermediate image screenaccording to the embodiment;

FIG. 40 is a diagram schematically illustrating a relationship among thethickness of the diffusion layer, a half-width at half-maximum angle ofa reflection light distribution angle, and the resolution of a videoimage formed on the reflection-type intermediate image screen;

FIG. 41 is a diagram illustrating, in a table format, results ofresearching influence of the distance between the diffusion layer and areflection surface on the resolution of a real image formed on a surfaceof the reflection-type intermediate image screen by changing thedistance from the diffusion layer to the reflection surface, andcalculated values of the resolution;

FIG. 42 is a graph illustrating a relationship between the distance fromthe diffusion layer to the reflection surface and the resolution of areal image formed on the surface of the reflection-type intermediateimage screen and a relationship between the distance from the diffusionlayer to the reflection surface, and the calculated values of theresolution;

FIG. 43 is a diagram schematically illustrating an example of athree-layer portion according to the embodiment;

FIG. 44 is a diagram schematically illustrating a screen holding unitaccording to the embodiment;

FIG. 45 is a top view illustrating a state where the screen holding unithas the three-layer portion installed therein;

FIG. 46 is a diagram illustrating a state where the screen holding unithas the three-layer portion installed therein;

FIG. 47 is a diagram illustrating a state where the screen holding unitis installed in the projection unit according to the embodiment; and

FIG. 48 is a diagram schematically illustrating another example of thethree-layer portion according to the embodiment.

DETAILED DESCRIPTION

Described below is an explanation of the embodiments of the presentinvention with reference to figures. Specific numerical values and thelike shown in the embodiments are shown merely for illustrative purposesto facilitate understanding of the invention and do not intend to limitthe scope of the present invention, unless otherwise noted. In thesubject specification and figures, elements having substantially thesame functions and structures shall be denoted by the same referencenumerals, and duplicative explanations will be omitted appropriately.Also, the illustration of elements that are not directly related to thepresent invention is omitted. An image display device according to theembodiment that is explained in the following is based on a displaycontrol device for a vehicle that is used in a vehicle. However, animage display device according to the embodiment is not limitedly usedfor a vehicle and can be also used in aircraft, game devices, amusementfacilities, and the like.

[Exterior Configuration of Image Display Device for Vehicle According toPresent Embodiment]

Using a head up display attached to a rear-view mirror provided to avehicle as an example for a display device for a vehicle according tothe present embodiment, an explanation is given regarding the exteriorconfiguration of the display device for a vehicle in reference to FIGS.1 and 2. FIG. 1 is a perspective view of a head up display 10 accordingto the present embodiment observed by means of a field of view directedfrom a rear-view mirror 600, to which this head up display 10 isattached, to a windshield (not shown) of a vehicle. FIG. 2 is aperspective view of the head up display 10 observed by means of a fieldof view directed from the windshield (not shown) to the rear-view mirror600. In the following explanations, directions that are shown asforward, backward, leftward, rightward, upward, and downward mean aforward direction, a backward direction, a leftward direction, arightward direction, a direction that is vertical to a road surface onwhich a vehicle is placed and that is directed from the surface to thevehicle, and a direction that is opposite to the direction,respectively.

The head up display 10 generates an image signal related to an imagedisplayed on a combiner 400 as a virtual image and is provided with asubstrate housing portion 100 housing a circuit substrate 111 (see FIG.10) that outputs the generated image signal to an optical unit 200. Animage signal output from an external device (not shown) such as anavigation device, a media reproduction device, or the like is input tothe circuit substrate 111, and the circuit substrate 111 is also capableof outputting the image signal to the optical unit 200 after performinga predetermined process on the signal that has been input. Thissubstrate housing portion 100 is connected to an attachment member 500described later (see FIG. 25), which is one of constituting elements ofthe head up display 10, and the rear-view mirror 600 is held by theattachment member 500. Thereby, the head up display 10 is attached tothe rear-view mirror 600. Details will be described later regarding eachof mechanisms related to the connection of the substrate housing portion100 and the attachment member 500 and to the holding of the rear-viewmirror 600 by the attachment member 500. Also, in order to facilitateexplanations and understanding of the entire configuration of the headup display 10, the descriptions of the attachment member 500 are omittedin FIGS. 1 and 2.

The head up display 10 is provided with the optical unit 200 to which animage signal output from the circuit substrate 111 is input. The opticalunit 200 is provided with an optical unit main body 210 and a projectionunit 300. The optical unit main body 210 houses a light source 231 andan image display element 240, which are described later, various opticallenses, and the like. The projection unit 300 houses various projectionmirrors and an intermediate image screen 360, which are described later.An image signal output by the circuit substrate 111 is projected from aprojection port 301 as image display light on the combiner 400 having aconcave shape via each of the devices of the optical unit main body 210and each of the devices of the projection unit 300. In the presentembodiment, a case where a liquid crystal on silicon (LCOS), which is areflection type liquid crystal display panel, is used as the imagedisplay element 240 is illustrated for example. However, a digitalmicromirror device (DMD) may be used as the image display element 240.In that case, the DMD is assumed to be formed by an optical system and adrive circuit according to a display element to which the DMD isapplied.

A user, who is a driver, recognizes projected image display light as avirtual image via the combiner 400. In FIG. 1, the projection unit 300is projecting image display light of a letter “A” on the combiner 400.Looking at the combiner 400, the user recognizes the letter “A” in sucha manner as if the letter were displayed, for example, 1.7 m to 2.0 mahead (in a forward direction of the vehicle). In other words, the usercan recognize a virtual image 450. In this case, a central axis of theimage display light projected on the combiner 400 from the projectionunit 300 is defined as a projection axis 320.

The optical unit 200 is configured such that the optical unit 200 isrotatable with respect to the substrate housing portion 100. A detailedexplanation thereof will be described later. Further, the head updisplay 10 according to the present embodiment is configured such thatthe projection unit 300 and the combiner 400 are changeable inattachment direction with respect to a predetermined surface of theoptical unit main body 210 and are detachable.

[Internal Configuration of Image Display Device for Vehicle According toPresent Embodiment: Optical System]

An explanation is now given regarding the internal configuration of thehead up display 10. FIGS. 3 and 9 are diagrams for explaining theinternal configuration of the optical unit 200 of the head up display 10described above. FIG. 3 is a diagram illustrating the internalconfiguration of the optical unit main body 210 and a part of theinternal configuration of the projection unit 300 along with an opticalpath related to image display light. FIG. 9 is a diagram illustratingthe internal configuration of the projection unit 300 and a part of theinternal configuration of the optical unit main body 210 along with anoptical path related to image display light projected onto the combiner400.

In reference to FIG. 3, an explanation is given regarding the internalconfiguration of the optical unit main body 210 and an optical pathrelated to image display light. The optical unit main body 210 isprovided with a light source 231, a collimate lens 232, a UV-IR(UltraViolet-Infrared Ray) cut filter 233, a polarizer 234, a fly-eyelens 235, a reflecting mirror 236, afield lens 237, a wire gridpolarization beam splitter 238, an image display light generator 244, ananalyzer 241, a projection lens group 242, and a heat sink 243.

The light source 231 consists of a light-emitting diode that emits whitelight or light in three colors: blue, green, and red. The heat sink 243for cooling heat generated along with emission of light is attached tothe light source 231. Light emitted by the light source 231 is changedto parallel light by the collimate lens 232. The UV-IR cut filter 233absorbs and removes ultraviolet light and infrared light from theparallel light passed through the collimate lens 232. The polarizer 234changes light that has passed through the UV-IR cut filter 233 toP-polarized light without disturbance. The fly-eye lens 235 then adjuststhe brightness of light that has passed through the polarizer 234 to beuniform. Linear polarized light that has passed through the UV-IR cutfilter 233 becomes P-polarized light in relation to an incident anglethereof to the wire grid polarization beam splitter 238.

The reflecting mirror 236 deflects the optical path of light that haspassed through each cell of the fly-eye lens 235 by 90 degrees. Lightreflected by the reflecting mirror 236 is collected by the field lens237. Light collected by the field lens 237 is radiated to the imagedisplay light generator 244 via the wire grid polarization beam splitter238 that transmits P-polarized light.

The image display light generator 244 generates image display lightbased on the light radiated via the wire grid polarization beam splitter238 and the image signal output by the circuit substrate 111 and emitsthe image display light as image display light. The image display lightemitted by the image display light generator 244 reenters the wire gridpolarization beam splitter 238 and becomes S-polarized light in relationto an incident angle thereof. The emitted S-polarized light is reflectedby the wire grid polarization beam splitter 238 and enters theprojection lens group 242 after changing the optical path and passingthrough the analyzer 241.

The image display light transmitted through the projection lens group242 exits the optical unit main body 210 and enters the projection unit300. A first projection mirror 351 provided on the projection unit 300then changes the optical path of the entering image display light.

[Configuration and Attachment of Image Display Light Generator]

Subsequently, an explanation is given regarding the internalconfiguration of the image display light generator 244 according to theembodiment and the attachment thereof.

FIG. 4 is an exploded perspective view of the image display lightgenerator 244 according to the embodiment. As shown in FIG. 3, the imagedisplay light generator 244 is installed vertically to an optical axisof light radiated from the light source 231 in an optical systemplacement unit 245 inside the optical unit main body 210. In acoordinate system shown in FIG. 4, a Z axis is an axis that is parallelto the optical axis, and a plane that is parallel to an X-Y plane formedby an X axis and a Y axis is a plane vertical to the optical axis. Thus,the image display light generator 244 is installed parallel to the X-Yplane.

Main optical members that constitute the image display light generator244 are: a quarter-wave plate 239, which performs conversion betweenlinear polarized light and circular polarized light; an aperture mask270, which defines an area irradiated with the circular polarized lightthat has passed through the quarter-wave plate 239; and an image displayelement 240, which reflects the circular polarized light that has passedthrough the aperture mask 270 based on an image signal of an image to bedisplayed so as to generate image display light. The image displayelement 240 is provided on a printed circuit board 250.

The aperture mask 270 is provided with an opening 271 for defining thearea irradiated with light and is formed such that the quarter-waveplate 239 can be attached covering a surface on the side of one end ofthe opening 271. The aperture mask 270 allows only light that is passingthe opening 271 to pass through and suppresses diffraction of light atthe opening 271. Therefore, a material that hardly transmits light evenwhen the material is thin is preferably used. As a specific example, theaperture mask 270 is formed using a metal material. Also, in order toprevent light reflected at a surface of the aperture mask 270 frombecoming stray light, the surface of the aperture mask 270 preferablyhas a color having a high light absorption rate. For example, thesurface of the aperture mask 270 is preferably subjected to alumitetreatment so as to have a black color.

The aperture mask 270 has wave-plate guides 272, which regulate anattachment position of the quarter-wave plate 239, at respectivepositions that face each other across the opening 271 on a surface onthe side where the quarter-wave plate 239 is attached. A semicircularplate 273 is provided on the quarter-wave plate 239 such that thesemicircular plate 273 fits along the wave-plate guides 272. After thequarter-wave plate 239 is attached to the aperture mask 270, thequarter-wave plate 239 can be rotated around a rotation axis that isparallel to the Z axis while being regulated by the wave-plate guides272.

More specifically, the quarter-wave plate 239 is attached to theaperture mask 270 by attachment screws 293 while the semicircular plate273 is held by holding springs 292. The holding springs 292 hold downthe quarter-wave plate 239 on the aperture mask 270 by elastic forcecaused by leaf springs. Therefore, even after the quarter-wave plate 239is attached to the aperture mask 270 by the attachment screws 293, thequarter-wave plate 239 can be rotated within a predetermined angle rangeby moving an adjustment tab 274 of the quarter-wave plate 239.

The image display element 240 attached to the printed circuit board 250reflects light that has transmitted through the quarter-wave plate 239and generates image display light based on the image signal generated bythe circuit substrate 111. More specifically, the image display element240 has a color filter of red, green, or blue for each pixel, and lightradiated to the image display element 240 is changed to a color thatcorresponds to each pixel, modulated by a liquid crystal compositionprovided on the image display element 240, and emitted toward adirection that is reversed by 180 degrees from an incident directionwhile being S-polarized image display light. Therefore, it is importantto perform position alignment such that the quarter-wave plate 239 andthe image display element 240 face each other.

The image display light generator 244 according to the embodiment isprovided with an attachment base 260 for attaching the aperture mask 270on which the quarter-wave plate 239 is attached and the printed circuitboard 250 on which the image display element 240 is attached. Theattachment base 260 is provided with an opening 261 serving as anoptical path, and the aperture mask 270 on which the quarter-wave plate239 is attached and the printed circuit board 250 on which the imagedisplay element 240 is attached are attached at respective positionsthat face each other across the opening 261 of the attachment base 260.More specifically, the aperture mask 270 provided with the quarter-waveplate 239 is attached such that the aperture mask 270 covers the entirefirst surface, which is a surface on the side of one end of the opening261 of the attachment base 260. Also, the printed circuit board 250 isattached such that the image display element 240 is positioned insidethe opening 261 of the attachment base 260 and such that the printedcircuit board 250 covers the entire second surface, which is a surfaceon the side of the other end of the opening 261 of the attachment base260. In the following, in the specification, the opening 271 of theaperture mask 270 is often referred to as “first opening,” and theopening 261 of the attachment base 260 is often referred to as “secondopening.”

The attachment base 260 is also provided with positioning pins 262 forregulating respective attachment positions of the aperture mask 270 andthe printed circuit board 250. The positioning pins 262 are providedsuch that the positioning pins 262 pass through the attachment base 260in a direction parallel to the optical axis. The aperture mask 270 andthe printed circuit board 250 are each provided with positioning holes251 and positioning holes 275 for inserting the positioning pins 262,respectively. By inserting the positioning pins 262 in the positioningholes 251 and the positioning holes 275, respective positions forattachment to the attachment base 260 are determined. The positioningpins 262 for regulating the respective attachment positions of theaperture mask 270 and the printed circuit board 250 each consist of asingle member. Thus, the accuracy of the attachment positions can beimproved.

The aperture mask 270 is fixed to the attachment base 260 by attachmentscrews 294. Similarly, the printed circuit board 250 is fixed to theattachment base 260 by attachment screws 290. Thereby, the quarter-waveplate 239 attached to the aperture mask 270 and the image displayelement 240 attached to the printed circuit board 250 can be easilyattached at respective predetermined attachment positions.

The image display element 240 attached to the printed circuit board 250is an electronic circuit and produces heat by energization. Thus, inorder to dissipate, via the printed circuit board 250, heat generated bythe image display element 240, the attachment base 260 is formed by ametal material. More specifically, in the area of a board surface of theprinted circuit board 250, a metal layer formed by a metallic foil thatserves as a ground of the image display element 240 is provided in atleast an area that comes in contact with the attachment base 260 whenattached to the attachment base 260.

FIG. 5A is a top view of the printed circuit board 250, on which theimage display element 240 is attached, viewed from an incident directionof light from the light source 231. FIG. 5B is a lateral view of theprinted circuit board 250 on which the image display element 240 isattached.

The attachment of the image display element 240 to the printed circuitboard 250 is performed according to the following steps. First, anadhesive agent 256 is applied on the printed circuit board 250 as shownin FIG. 5B. Using the adhesive agent 256, the image display element 240is mounted at a predetermined position of the printed circuit board 250.When the image display element 240 is mounted on the printed circuitboard 250, the image display element 240 is electrically connected onthe printed circuit board 250 by a bonding wire 254. After that, aliquid crystal seal 253 is applied, and a counter glass 252 is attached.A protection resin 255 that protects the last bonding wire 254 isapplied. A connector 257 connects a flexible cable 246 (described later)and transmits, to the image display element 240, an image signalgenerated by the circuit substrate 111.

As shown in FIG. 5A, in the printed circuit board 250, the positioningholes 251 for regulating the position for the attachment to theattachment base 260 are provided at two parts. By allowing thepositioning pins 262 to pass through the respective positioning holes251, the image display element 240 attached to the printed circuit board250 can be easily attached at a predetermined attachment position of theattachment base 260.

In general, a printed circuit board is often formed by a phenol resin,an epoxy resin, or the like. Therefore, the thermal conductivity can beincreased by providing a metal layer having thermal conductivity that ishigher than the above-stated resins on the area that comes in contactwith the attachment base 260. In FIG. 5A, an area of the surface of theprinted circuit board 250 that is shown by diagonal lines represents anarea that comes in contact with the attachment base 260 when attached tothe attachment base 260. In the printed circuit board 250 according tothe embodiment, the metal layer that serves as the ground of the imagedisplay element 240 is provided in the area shown by diagonal lines inFIG. 5A. Since the attachment base 260 is formed by a metal material,the heat of the image display element 240 can be more easily dissipated,and the attachment base 260 can be caused to function also as the groundof the image display element 240.

Meanwhile, when the attachment base 260 is formed by a metal material,light that is reflected at a surface of the attachment base 260 maybecome stray light. Therefore, as in the case of the aperture mask 270,the surface of the aperture mask 270 preferably has a color having ahigh light absorption rate in order to prevent stray light. For example,the surface of the aperture mask 270 is preferably subjected to alumitetreatment so as to have a black color.

FIG. 6 is a perspective view of the image display light generator 244according to the embodiment viewed toward an emission direction of imagedisplay light. As shown in FIG. 6, the side of the opening 261 of theattachment base 260 on which image display light generated by the imagedisplay element 240 is incident is closed by attaching to the attachmentbase 260 the printed circuit board 250 on which the image displayelement 240 is attached.

FIG. 7 is a perspective view of the image display light generator 244according to the embodiment viewed from an incident direction of lightfrom the light source 231. As shown in FIG. 7, the side of the opening261 of the attachment base 260 in the incident direction of light fromthe light source 231 is closed by attaching to the attachment base 260the aperture mask 270 on which the quarter-wave plate 239 is attached.More specifically, the aperture mask 270 is fixed to the attachment base260 by the attachment screws 294, and the quarter-wave plate 239 is helddown on the aperture mask 270 by elastic force of the holding springs292.

As described above, the attachment base 260 attaches the quarter-waveplate 239 and the aperture mask 270 to the first surface, which is thesurface on the side of one end of the opening 261, and attaches theprinted circuit board 250, on which the image display element 240 isattached, to the second surface, which is the surface on the side of theother end of the opening 261, thereby covering both ends of the opening261. More specifically, the quarter-wave plate 239 and the aperture mask270 are attached to the first surface of the surfaces of the attachmentbase 260, which is the surface on the side of the incident direction oflight from the light source 231. The printed circuit board 250 on whichthe image display element 240 is attached is attached to the secondsurface of the surfaces of the attachment base 260, which is the surfaceon the side of the emission direction of light from the light source231.

Thereby, a dust-proof structure that prevents the entry of dust and thelike inside the opening 261 can be achieved without using dedicatedcomponents for dust proofing at the opening 261 of the attachment base260. In particular, in order to prevent the entry of dust or dirt insidethe opening 261, a process of attaching the quarter-wave plate 239, theaperture mask 270, and the printed circuit board 250 to the opening 261of the attachment base 260 is preferably performed inside a clean room.

Also, as described above, the metal layer that serves as the ground ofthe image display element 240 is provided in the area on the printedcircuit board 250 that comes in contact with the attachment base 260.Thus, the attachment base 260 can dissipate heat from the printedcircuit board 250 as well as functioning as the ground of the imagedisplay element 240.

Dust or dirt may attach to a surface of the quarter-wave plate 239 onthe side of the incident direction of light from the light source 231.However, the surface of the quarter-wave plate 239 on the side of theincident direction of light from the light source 231 is apart from theimage display element 240, which generates image display light, at leastby an amount of the thickness of the attachment base 260. Therefore,there is an effect that dust or dirt attached to the surface of thequarter-wave plate 239 on the side of the incident direction of lightfrom the light source 231 is defocused and becomes less prominent.

As described above, the quarter-wave plate 239 is a device thatconverts, when linear polarized light is made incident, the linearpolarized light into circular polarized light and that converts, whencircular polarized light is made incident, the circular polarized lightinto linear polarized light. The efficiency of the conversion of thequarter-wave plate 239 depends on the rotation angle of the quarter-waveplate 239 with respect to light made incident on the quarter-wave plate239. Thus, in order to optimize the conversion efficiency of thequarter-wave plate 239, the rotation angle of the quarter-wave plate 239with respect to the optical axis is adjusted.

As shown in FIG. 7, the quarter-wave plate 239 is attached to theaperture mask 270 by elastic force of the holding springs 292. Thus, bymoving the adjustment tab 274, the quarter-wave plate 239 can be rotatedusing the optical axis as the rotation axis thereof. Thereby, theconversion efficiency of the quarter-wave plate 239 can be optimallyadjusted, for example, at the time of assembly in a production line.Also, by fixing the quarter-wave plate 239 to the aperture mask 270after optimizing the conversion efficiency, the conversion efficiencycan be maintained at the optimal state after the shipping of a product.

As described above, by attaching the aperture mask 270, on which thequarter-wave plate 239 is attached, to one end of the opening 261 andattaching the printed circuit board, on which the image display element240 is attached, to the other end of the opening 261, the positionalignment between the quarter-wave plate 239 and the image displayelement 240 can be achieved. On the other hand, since the image displaydevice according to the embodiment is a head up display, adjustment ismade such as imaging, at a predetermined position of the intermediateimage screen 360, image display light generated by the image displaylight generator 244.

FIG. 8 is a diagram for explaining adjustment of an attachment positionto the optical system placement unit 245 in the image display lightgenerator 244 according to the embodiment. As shown in FIG. 8, theattachment base 260 and the aperture mask 270 are provided withattachment holes 263 for allowing the attachment screws 290 to passthrough. As shown in FIG. 8, the attachment holes 263 are provided atthree places, and by screwing using the three attachment screws 290, theimage display light generator 244 is installed in the optical systemplacement unit 245.

The diameter of the attachment holes 263 serving as screw holes of theattachment screws 290 is larger than the screw diameter of theattachment screws 290, securing clearance at an installation position ofthe image display light generator 244. More specifically, the diameterof the attachment holes 263 is 1.2 to 1.3 times larger than the screwdiameter of the attachment screws 290.

As described, by screwing the image display light generator 244 by theattachment screws 290 having a screw diameter that is smaller than thediameter of the attachment holes 263 by a predetermined rate, the imagedisplay light generator 244 becomes slidable in all directions on aplane that is parallel to the X-Y plane shown in FIG. 4, and theattachment position thereof becomes freely adjustable at the time of theinstallation of the image display light generator 244 in the opticalsystem placement unit 245. As a result, the installation position of theimage display light generator 244 can be adjusted such that imagedisplay light generated by the image display light generator 244 isimaged at a predetermined position of the intermediate image screen 360(e.g., the central part of the intermediate image screen 360). In thissense, the attachment holes 263 and the attachment screws 290 functionas installation position adjusters of the attachment base 260.

As shown in FIG. 8, a C chamfering process is performed on theattachment base 260 and the aperture mask 270, and an area where theattachment base 260 and the aperture mask 270 come in contact with eachother is smaller compared to the area before the C chamfering process isperformed. Therefore, mechanical stress produced in both the attachmentbase 260 and the aperture mask 270 when the aperture mask 270 isattached to the attachment base 260 can be alleviated. The image displaylight generator 244 is screwed by the three attachment holes 263provided on the attachment base 260 and the aperture mask 270. Thus,stress caused by deformation produced in the attachment base 260 and theaperture mask 270 mechanically and due to heat can be suppressed.

As explained above, according to a head up display according to theembodiment of the present invention, a technology can be provided thatis for achieving both dust proofing and heat dissipation of an opticalcomponent and that allows for positioning of the optical component to befacilitated.

In particular, by fitting the aperture mask 270 provided with thequarter-wave plate 239 and the printed circuit board 250 provided withthe image display element 240 to the positioning pins 262 provided whilepenetrating through the attachment base 260, the positioning of thequarter-wave plate 239, the aperture mask 270, and the image displayelement 240 can be facilitated, and the assembly can be simplified.

Also, by closing the opening 261 of the attachment base 260 by theaperture mask 270 provided with the quarter-wave plate 239 and theprinted circuit board 250 provided with the image display element 240, adust-proofing structure can be built without separately attaching amember for dust proofing. Further, by forming the attachment base 260 bya metal material, the attachment base 260 can serve as the ground of theprinted circuit board 250, and heat produced by the printed circuitboard 250 can be efficiently dissipated.

In the image display light generator 244 formed by attaching thequarter-wave plate 239, the aperture mask 270, and the image displayelement 240 to the attachment base 260, clearance for adjustment isprovided that allows for position adjustment at the time of theattachment to the optical system placement unit 245. Therefore, thepositioning of image display light generated by the image display lightgenerator 244 and the intermediate image screen 360 can be facilitated,and the assembly can be simplified.

Subsequently, in reference to FIG. 9, an explanation is given regardingthe internal configuration of the projection unit 300 and an opticalpath related to image display light. The projection unit 300 is providedwith the first projection mirror 351, a second projection mirror 352,and the intermediate image screen 360.

As described above, the optical path of the image display light that haspassed through the wire grid polarization beam splitter 238, theanalyzer 241, and the projection lens group 242 provided in the opticalunit main body 210 is changed to an optical path heading toward thecombiner 400 by the first projection mirror 351 and the secondprojection mirror 352. In the meantime, a real image based on the imagedisplay light reflected by the second projection mirror 352 is formed onthe intermediate image screen 360. The image display light related tothe real image formed on the intermediate image screen 360 istransmitted through the intermediate image screen 360 and projected onthe combiner 400. As described above, the user recognizes a virtualimage related to this projected image display light in the forwarddirection via the combiner 400.

An internal configuration such as the one described above allows for theuser to visually recognize a virtual image based on an image signaloutput from the circuit substrate 111 while overlapping the virtualimage on the real landscape via the combiner 400.

[Internal Configuration of Image Display Device for Vehicle According toPresent Embodiment: Details of Internal Configuration of Optical Unit200]

The optical unit 200 is configured such that the optical unit 200 isrotatable with respect to the substrate housing portion 100.Subsequently, in reference to FIG. 10, a detailed description is maderegarding the internal configuration of the optical unit 200 and thesubstrate housing portion 100, mainly regarding an area near a partwhere the optical unit 200 and the substrate housing portion 100 areconnected.

FIG. 10 is a diagram illustrating a part of the inside of the opticalunit 200 and a part of the inside of the substrate housing portion 100.In FIG. 10, the area near the part where the optical unit 200 and thesubstrate housing portion 100 are connected is mainly shown. An opticalsystem placement unit 245 provided in the optical unit 200 housesvarious devices except for the above-described heat sink 243. The heatsink 243 and a space 248 are provided inside the optical unit 200 nearthe part where the optical unit 200 is connected to the substratehousing portion 100 on the side of the substrate housing portion 100 ofthe optical system placement unit 245.

The circuit substrate 111 electrically controls the image displayelement 240 and the light source 231 housed in the optical systemplacement unit 245. The circuit substrate 111 and the image displayelement 240 housed in the optical system placement unit 245 areconnected through a flexible cable 246, which is a wiring. The flexiblecable 246 is shown as an example in the figure, and a wiring thattransmits an electrical signal of a flexible substrate or the like canbe used. In the optical unit 200, an optical unit side opening 247 isformed on a surface of a housing of the optical unit 200. In thesubstrate housing portion 100, a substrate housing side opening 112 isformed on a surface of a housing of the substrate housing portion 100.The flexible cable 246 connects the circuit substrate 111 and the imagedisplay element 240 through the optical unit side opening 247 and thesubstrate housing side opening 112. The flexible cable 246 preferablyhas a length that allows the substrate housing portion 100 and theoptical unit 200 to rotate freely.

FIG. 11 is a diagram illustrating a state where the above-stated heatsink 243 and the flexible cable 246 are removed regarding the part ofthe inside of the optical unit 200 and the part of the inside of thesubstrate housing portion 100 shown in FIG. 10.

The optical unit side opening 247 and the substrate housing side opening112 each have a shape having two sides facing each other that diverge ata predetermined angle and, as an example, are formed in an approximatelyfan-like shape having a predetermined angle. This allows for a reductionin force applied to the flexible cable 246 by a housing relating to asurface of the optical unit 200 on which the optical unit side opening247 is provided and by a housing relating to a surface of the substratehousing portion 100 on which the substrate housing side opening 112 isprovided when the optical unit 200 is rotated with respect to thesubstrate housing portion 100. Therefore, breakage or cutting of theflexible cable 246 by the housings due to the rotation can be prevented.

Also, as described above, the space 248 is provided near the part wherethe substrate housing portion 100 is connected in the optical unit 200,and the flexible cable 246 is mainly housed by this space 248 in theoptical unit 200. By providing this space 248, the length of theflexible cable can be ensured with a margin. Therefore, tension appliedto the flexible cable 246 can be reduced when the optical unit 200 isrotated with respect to the substrate housing portion 100. Thus,breakage or cutting of the flexible cable 246 by the tension due to therotation can be prevented.

The optical unit 200 and the substrate housing portion 100 are connectedby a hinge 113, which is a rotating member serving as a rotation axis ofthe rotation of each other and a rotation lock mechanism 114, whichrestricts the angle range of the rotation. The optical unit 200 rotateswith respect to the substrate housing portion 100 by a predeterminedangle around this hinge 113. In the present embodiment, the hinge 113 isused in this case. However, another rotating member can be used.

The substrate housing side opening 112 of the substrate housing portion100 and the optical unit side opening 247 of the optical unit 200 areformed in an approximately fan-like shape as described above. When thesubstrate housing portion 100 rotates with respect to the optical unit200, an opening that is formed by both the substrate housing sideopening 112 and the optical unit side opening 247 and that is for theflexible cable 246 to pass through is narrowed. However, an opening thatis sufficient for the flexible cable 246 to pass through is maintainedin the angle range restricted by the rotation lock mechanism 114 by theapproximately fan-like shape of the substrate housing side opening 112and the optical unit side opening 247.

The above-described shape of the substrate housing side opening 112 andthe optical unit side opening 247 is shown for illustrative purposes. Aslong as the substrate housing side opening 112 and the optical unit sideopening 247 have a shape that does not cause breakage or the like of theflexible cable 246 due to the rotation, the shape can be any form. Forexample, only one of the substrate housing side opening 112 and theoptical unit side opening 247 may be formed in a shape having two sidesfacing each other that diverge at a predetermined angle such that a loadis prevented from being imposed on the flexible cable 246.

As described above, the head up display 10 is configured such that theoptical unit 200 and the substrate housing portion 100 are rotatablearound the hinge 113. The combiner 400 is provided on the optical unit200, and the substrate housing portion 100 is attached to the rear-viewmirror 600 by the attachment member 500. By employing such aconfiguration described above, the user can perform adjustment of theobservation angle of the rear-view mirror and adjustment of theobservation angle of the combiner 400 independently from each other.Therefore, the user can adjust the visually-recognizable angle of thecombiner 400 as well as adjusting the rear-view mirror 600 at an anglethat allows for an area behind the vehicle to be properly checked so asto perform recognition of a proper distortionless image (virtual image).

The combiner 400 of the head up display 10 is formed such that thecurvature thereof in a specific direction becomes smaller as a distancefrom the projection port 301 becomes increased, as described later indetail. This structure allows for the presentation of an image withlittle distortion even when the combiner 400 is downsized. As describedabove, since the optical unit 200 is formed such that the optical unit200 is rotatable with respect to the substrate housing portion 100, theuser can adjust the visually-recognizable angle of the combiner 400while maintaining a positional relationship between the projection port301 of the optical unit 200 and the combiner 400. Therefore, accordingto the head up display 10, the user can adjust the visually-recognizableangle of the combiner 400 while presenting an image with littledistortion on the combiner 400.

Also, by providing the space 248 for housing the flexible cable 246ensured with a length with a margin in the optical unit 200, rotation ofthe optical unit 200 with respect to the substrate housing portion 100is achieved freely. Thereby, the user can properly adjust the respectiveobservation angles, and breakage or cutting of the flexible cable 246 bytension due to the rotation can be prevented.

Further, by allowing the substrate housing side opening 112 and theoptical unit side opening 247 of the optical unit 200 to have theabove-stated approximately fan-like shape, breakage or cutting of theflexible cable 246 caused by the respective housing exterior walls ofthe optical unit 200 and the substrate housing portion 100 due to therotation of the optical unit 200 with respect to the substrate housingportion 100 can be prevented, and the user can properly adjust therespective observation angles.

Also, as shown in FIG. 3, the optical path of the image display light isbent twice in the direction of 90 degrees by using the reflecting mirror236 and the wire grid polarization beam splitter 238 in the presentembodiment. The image display light is then emitted to the projectionunit 300 in a direction opposite to the direction of light emission inthe light source 231. By making the path of the image display light tobe U-shaped in this way, the flexible cable 246 can be wired such thatthe flexible cable 246 and the light source 231 are not located close toeach other (see FIG. 10). With this, noise caused by an electromagneticwave generated by the light source 231 can be prevented from being mixedin an image signal, and breakage of the flexible cable 246 caused byheat generated by the light source 231 can be also prevented. Further,since the heat sink 243 installed close to the light source 231 isplaced away from the flexible cable 246, the space 248 for housing theflexible cable 246 can be provided.

[Angle Adjustment Using Hinge]

A description is given in detail regarding the rotation of the opticalunit 200 with respect to the substrate housing portion 100 describedabove. FIG. 12 is a lateral view of the head up display 10 attached tothe rear-view mirror 600. As shown in this figure, the rear-view mirror600 is normally directed to the driver side so that the driver canvisually check behind the vehicle. In other words, a driver rarelydrives while a mirror surface 602 of the rear-view mirror 600 isperpendicular to a vehicle bottom surface or a traveling road surface.Normally, a driver tilts the direction of the rear-view mirror 600 sothat the mirror surface 602 of the rear-view mirror 600 has an anglewith respect to a surface perpendicular to the vehicle bottom surface orthe like. Therefore, when the head up display 10 is attached to therear-view mirror 600, the substrate housing portion 100 also has anangle with respect to a surface that is parallel to the vehicle bottomsurface or the like in association with the inclination of the rear-viewmirror 600.

As a result of performing an experiment for the recognition of a virtualimage presented in many vehicles and to various users by the combiner400, the inventor of the subject application has confirmed by theexperiment that, in most cases, an angle formed by the mirror surface602 and a reference surface 212 of the optical unit main body 210becomes approximately 100 degrees by adjusting the respective angles ofthe combiner 400 and the optical unit 200 such that the combiner 400 andthe optical unit 200 are located at a position where the user canrecognize the virtual image without distortion when the head up display10 is placed such that the longitudinal direction of the rear-viewmirror 600 and the longitudinal direction of the substrate housingportion 100 are in the same direction.

The “reference surface” of the optical unit main body 210 in this caseis an angle measurement reference surface used as a reference formeasuring the inclination of the optical unit main body 210 with respectto the mirror surface 602 of the rear-view mirror 600. An example of thereference surface 212 is a plane including an optical axis of theoptical unit main body 210 or a plane parallel to the plane. Anotherexample of the reference surface 212 is a first main body surface 221,which is a lower surface when the head up display 10 is attached for aright steering wheel, or a second main body surface 222, which is asurface that is opposite to the first main body surface 221, or a planethat is parallel to those surfaces. The “reference surface” of theoptical unit main body 210 may be set to be a reference surface of theoptical unit 200.

In view of the above experimental result, the head up display 10according to the embodiment is designed such that an optimal video imagewithout distortion can be presented when the angle formed by the mirrorsurface 602 and the reference surface 212 is a predetermined referenceangle under the condition where the head up display 10 is attached tothe rear-view mirror 600 using the attachment member 500, attachmentplates 571 and 581, and the like such that the longitudinal direction ofthe rear-view mirror 600 and the longitudinal direction of the substratehousing portion 100 are in the same direction. More specifically, anoptical unit forming the optical system of the head up display 10 isdesigned such that an optical video image can be presented under theabove-stated condition.

The “optical unit forming the optical system of the head up display 10”in this case is a system that generates and projects image display lightbased on an image signal output by the circuit substrate 111 housed inthe substrate housing portion 100. More specifically, the systemrepresents all or a predetermined part of the light source 231, thecollimate lens 232, the UV-IR cut filter 233, the polarizer 234, thefly-eye lens 235, the reflecting mirror 236, the field lens 237, thewire grid polarization beam splitter 238, the quarter-wave plate 239,the analyzer 241, and the projection lens group 242 in the optical unitmain body 210, the first projection mirror 351, the second projectionmirror 352, and the intermediate image screen 360 in the projection unit300, and the combiner 400.

Also, the “predetermined reference angle” is an angle formed by themirror surface 602 and the reference surface 212 and an angle assumed asa standard for design at the time of the optical designing of the headup display 10. The “predetermined reference angle” may be determined byan experiment so that an optimal video image without distortion can bepresented in many vehicles and to various users. An example of thepredetermined reference angle is an obtuse angle and is morespecifically 100 degrees. Also, the “predetermined reference angle” isshown using Ø in FIG. 12.

As described, in the head up display 10 according to the embodiment, anoptical part forming an optical system is designed using, as areference, a condition when the angle formed by the mirror surface 602and the reference surface 212 becomes the reference angle. Thus, theoptical designing is optimally achieved in accordance with theinclination of the rear-view mirror 600 that can be expected under anormal state of usage. When the head up display 10 according to theembodiment is attached such that an optimal video image withoutdistortion can be presented in many vehicles and to various users, theoptical unit 200 is held near horizontal in most cases. Since theoptical unit 200 does not face the direction of the user, a feeling ofoppression the user, who is the driver, has can be reduced.

The substrate housing portion 100 attached via the attachment member 500(not shown) is fixedly installed on the rear-view mirror 600 directed tothe user as described above in FIG. 12. Therefore, the same change indirection as in the rear-view mirror 600 is made to the substratehousing portion 100. On the other hand, as described above, the opticalunit 200 including the projection unit 300 and the combiner 400 arerotatable in an integral manner by the hinge 113 with respect to thesubstrate housing portion 100. Therefore, regardless of an angle ofadjustment of the rear-view mirror 600, the driver can adjust thecombiner 400 to a visually-recognizable position without creatingdistortion in an image (virtual image) projected onto the combiner 400.

FIG. 13 is a diagram of the head up display 10 attached to the rear-viewmirror 600 viewed by means of a field of view from the side of themirror surface 602 of the rear-view mirror 600. As shown in the figure,a rotation surface of the hinge 113, which is a boundary surface betweenthe substrate housing portion 100 and the optical unit 200 formed by therotation of the hinge 113, is a surface that is perpendicular to themirror surface 602 and that is parallel to the projection axis 320 andis therefore located at a position where the rotation surface does notcross the rear-view mirror 600. Therefore, the optical unit 200 and thecombiner 400 can be rotated in an integral manner without coming intocontact with the rear-view mirror 600 while the substrate housingportion 100 is fixed to the rear-view mirror 600.

FIGS. 14 and 15 are diagrams illustrating a space where an image(virtual image) projected on the combiner 400 is visually recognizableand diagrams for explaining a change in the driver's direction ofobserving the optical unit 200 and the combiner 400 rotated by theabove-described hinge 113. For example, when a head up display 10attached to the same vehicle is used by a driver A and a driver B whoseeye level is higher than that of the driver A, an angle of adjustment bythe hinge 113 made for use by the driver A is angle Ø1 as shown in FIG.14. This angle allows the driver A to visibly recognize the image(virtual image) projected on the combiner 400 without distortion. On theother hand, an angle of adjustment by the hinge 113 made for use by thedriver B is Ø2, which is larger than the angle Ø1, as shown in FIG. 15.This angle Ø2 allows the driver B to visually recognize the image(virtual image) projected on the combiner 400 without distortion. Thisrotation of the hinge 113 from the angle Ø1 to the angle Ø2 changes aposition at which the image is displayed as a virtual image by thecombiner 400 in a direction parallel to a straight line formed mainly bythe rotation surface and the mirror surface 602 of the rear-view mirror600.

Therefore, even when the head up display 10 according to the presentembodiment is installed in a small space in a vehicle, both theprojection direction of image display light from the projection unit 300and the combiner 400 on which the image display light is projected canbe adjusted in a space-saving manner. Also, since only the optical unit200 and the combiner 400 can be moved in an integral manner withoutmoving the entire head up display 10, a space that allows a displayimage to be visually recognized can be easily adjusted.

[Shape of Combiner According to Present Embodiment]

Subsequently, the shape of the combiner 400 is described in detail. FIG.16 is a perspective view illustrating the shape of the combiner 400.Image display light projected along the projection axis 320 from theprojection port 301 is reflected at a projection position 322 on areflection surface 410 of the combiner 400. The image display light thathas been reflected travels along a reflection axis 330 and reaches theuser, who is a driver. The user visually recognizes the image displaylight projected from the projection port 301 as a virtual image that isviewed in a visual-line direction 340, which is the forward direction ofthe combiner 400.

If the reflection surface 410 of the combiner 400 is formed by aspherical surface, a virtual image of image display light presented to auser E may be distorted, and the visibility may thus be lowered. Inparticular, if a distance between the projection port 301 and thecombiner 400 is shortened in order to realize a small-sized imagedisplay device of a rear-view mirror mounting type, the curvature of thereflection surface 410 of the combiner 400 needs to be large in order topresent a large virtual image to a user. When the curvature of thereflection surface 410 is large, spherical aberration and astigmatismbecome increased, and a virtual image presented to the user is morelikely to be distorted. Thus, it is difficult to present high-definitionand highly visible image display light.

Thus, in order to reduce distortion of an image and deterioration indefinition due to aberration, the reflection surface 410 of the combiner400 according to the present embodiment is formed by an asphericalsurface. For example, the reflection surface 410 is formed by a biconicsurface expressed by Expression (1). In Expression (1), a z-axisdirection represents the visual-line direction 340 of the user, and anx-axis direction and a y-axis direction represent the horizontaldirection and the vertical direction that are perpendicular to thevisual-line direction 340, respectively. Note that, for the sake of easeof explanation, the position of the origin of the xyz axis is at avertex of the biconic surface that forms the reflection surface 410 ofthe combiner 400.

$\begin{matrix}{Z = \frac{{c_{x}x^{2}} + {c_{y}y^{2}}}{1 + \sqrt{1 - {\left( {1 + k_{x}} \right)c_{x}^{2}x^{2}} - {\left( {1 + k_{y}} \right)c_{y}^{2}y^{2}}}}} & {{Expression}\mspace{14mu}(1)}\end{matrix}$c_(x): Curvature in x direction (reciprocal of curvature radius in xdirection)c_(y): Curvature in y direction (reciprocal of curvature radius in ydirection)k_(x): Conic constant in x directionk_(y): Conic constant in y direction

By changing the value of a conic constant, the shape of the biconicsurface expressed by Expression (1) can be changed to a hyperboloidsurface, a paraboloid surface, an ellipsoid surface, a sphericalsurface, or the like. For example, when the conic constant in the xdirection satisfies a relationship of k_(x)<−1, the shape of the biconicsurface in an x-z plane (y=0) becomes a hyperbolic curve. Similarly, theshape becomes parabolic when k_(x)=−1, ellipsoidal when −1<k_(x)<0,circular when k_(x)=0, and an oblate ellipsoid k_(x)>0. The samerelationship is also established for the biconic constant k_(y) in the ydirection. By changing the biconic constant k_(y) in the y direction,the shape of the biconic surface in a y-z plane (x=0) changes to ahyperbolic curve, a parabola, an ellipsoid line, or the like.

A description is made in the following regarding a specific shape of thecombiner 400 when the reflection surface 410 is formed by a biconicsurface. As shown in FIG. 16, the combiner 400 according to theembodiment is provided such that a vertex position 422 of the biconicsurface forming the reflection surface 410 is located at a position thatis moved in the +y direction from the projection position 322, which isthe point of intersection of the projection axis 320 and the reflectionsurface 410. The combiner 400 is provided such that the vertex position422 is located at a position with the closest distance from theprojection port 301. A first curve 431 indicates a line of intersectionof the reflection surface 410 and the y-z plane, and a second curve 432indicates a line of intersection of the reflection surface 410 and thex-z plane in FIG. 16.

FIG. 17 is a lateral view illustrating the combiner 400 shown in FIG.16. FIG. 17 illustrates a cross section of the combiner 400 cut alongthe y-z plane passing through the vertex position 422. Therefore, thefirst curve 431, which represents the reflection surface 410 cut alongthe y-z plane, is determined by a function obtained when x=0 inExpression (1) where the vertex position 422 is set to be the point oforigin.

The vertex position 422 of the reflection surface 410 passing throughthe first curve 431 is provided at a position that is moved in the +ydirection from the projection position 322, which is the point ofintersection of the reflection surface 410 and the projection axis 320at this time. In other words, the vertex position 422 is arranged suchthat a reference axis 420 of the biconic surface that orthogonallyintersects the vertex position 422 has a positional relationship wherethe reference axis 420 is decentered in the y direction with respect tothe visual-line direction 340 from a viewpoint E of the user. Therefore,image display light projected along the projection axis 320 from theprojection port 301 is reflected at the reflection surface 410, changingthe direction thereof, and the user who is in the direction of thereflection axis 330 can visually recognize reflected light thereof.

Instead of having a shape that is symmetrical in the y-axis directionusing the vertex position 422 of the biconic surface as a referenceaxis, the reflection surface 410 of the combiner 400 is formed to have ashape that is asymmetrical using the lower half of the biconic surface.Thereby, the combiner 400 can be made to be smaller compared to a casewhere the upper half of the biconic surface is included in thereflection surface.

FIG. 18 is a top view illustrating the combiner 400 shown in FIG. 16.FIG. 18 illustrates a cut cross section of the combiner 400 cut alongthe x-z plane passing through the vertex position 422. Therefore, thesecond curve 432, which represents the reflection surface 410 cut alongthe x-z plane, is determined by a function obtained when y=0 inExpression (1) where the vertex position 422 is set to be the point oforigin.

The vertex position 422 of the reflection surface 410 passing throughthe second curve 432 is provided at a position where the vertex position422 and the projection position 322, which is the point of intersectionof the reflection surface 410 and the projection axis 320, have the samex coordinate at this time. In other words, the vertex position 422 isarranged such that the visual-line direction 340 from the viewpoint E ofthe user and the reference axis 420 of the biconic surface thatorthogonally intersects the vertex position 422 have a positionalrelationship where the visual-line direction 340 and the reference axis420 are concentric with respect to the x direction. Therefore, the useris capable of visually recognizing reflected light with less aberrationwith respect to the x direction.

In the combiner 400 according to the present embodiment, a biconicsurface expressed by Expression (1) that has a curved shape of any oneof a hyperbolic curve, a parabola, and an ellipsoid line in the xdirection is used. Since the conic constant satisfies a relationship ofk_(x)<0, the curvature in the x direction of the biconic surface becomesmaximum at x=0, which is the point of origin, and the curvature in the xdirection becomes smaller as the absolute value of the x coordinatebecomes larger at this time. In other words, with respect to the x-axisdirection along the reflection surface of the combiner 400, thecurvature of the biconic surface in the x direction becomes smaller as adistance from the vertex position 422, which has the shortest distanceto the projection port, becomes larger.

Similarly, any one of a hyperbolic curve, a parabola, and an ellipsoidline is also used for a curved shape in the y direction. Since the conicconstant satisfies a relationship of k_(y)<0, the curvature in the ydirection of the biconic surface becomes maximum at y=0, which is thepoint of origin, and the curvature in the y direction becomes smaller asthe absolute value of the y coordinate becomes larger at this time. Inother words, with respect to the y-axis direction along the reflectionsurface of the combiner 400, the curvature of the biconic surface in they direction becomes smaller as a distance from the vertex position 422,which has the shortest distance to the projection port, becomes larger.

Therefore, with respect to a specific direction along the reflectionsurface 410, the curvature in the specific direction becomes smaller asa distance from the vertex position 422 becomes larger. The biconicsurface that forms the reflection surface 410 is provided such that thevertex position 422 is located at a position that is closest to theprojection port 301. Thus, the curvature with respect to a specificdirection becomes maximum at the vertex position 422, which has theshortest distance from the projection port 301, and becomes smaller as adistance from the projection port 301 becomes larger in the specificdirection.

FIG. 19 is a lateral view illustrating an optical path of image displaylight presented to a user via the combiner. Projection light 324projected from the projection port 301 is reflected at a reflectionsurface of the combiner 400. Projection light 334 that has beenreflected reaches a user E and visually recognized as a virtual image450. By forming the reflection surface 410 by a biconic surface, thevirtual image 450 with little distortion can be presented even whenimage display light having a two-dimensional size is projected at thistime. In particular, even when the curvature of the reflection surface410 of the combiner 400 is increased in order to shorten a distancebetween the projection port 301 and the combiner 400, the distortion ofthe virtual image 450 presented to the user can be suppressed, and thevirtual image 450, which is high-definition and highly visible, can bepresented by forming the reflection surface 410 by a biconic surface.

FIG. 20 is a lateral view illustrating an optical path of image displaylight when a viewpoint of the user, who is a driver, is moved.Viewpoints E1 through E3 of the user, who is the driver, changesdepending on the height or the seating position of the driver. Even whenthe viewpoint of the user moves, it is convenient for the user if theuser is able to visually recognize a virtual image 450 withoutdistortion as long as the movement is in a certain moving range. Theinventors have found that by shaping the first curve 431, which is theshape of a biconic surface in the y direction, to be a hyperbolic curveclose to a parabola, a virtual image 450 with little distortion can bepresented even when the viewpoint of the user moves up and down.

In order for the first curve 431 to be a hyperbolic curve close to aparabola, the value of the conic constant k_(y) in the y direction isset to be −1 or less and is close to −1. For example, the value of theconic constant k_(y) is a value that satisfies a relationship of−1>k_(y)>−2. A specific value thereof is desirably determined byexperiments according to the size of the virtual image 450, a positionalrelationship among the projection port 301, the combiner 400, and thevirtual image 450, the degree of decentering of the projection axis 320and the reference axis 420, and the like.

FIG. 21 is a top view illustrating an optical path of image displaylight presented to the user via the combiner. In the same way as in FIG.20, projection light projected from the projection port 301 is reflectedby the combiner 400, and reflected light thereof then reaches a left eyeE1 and a right eye E2 of the use and is visually recognized as a virtualimage 450. A virtual image visually recognized by the left eye E1 and avirtual image visually recognized by the right eye E2 are both desirablypresented as virtual images with little distortion. The inventors havefound that by shaping the second curve 432, which is the shape of abiconic surface in the x direction, to be an ellipsoid line close to aparabola, a virtual image 450 with little distortion can be presentedeven when respective viewpoints in the left and right directions aredifferent.

In order for the second curve 432 to be an ellipsoid line close to aparabola, the value of the conic constant k_(x) in the x direction isset to be −1 or greater and is close to −1. For example, the value ofthe conic constant k_(x) is a value that satisfies a relationship of−1<k_(x)<−0.5. A specific value thereof is desirably determined byexperiments according to the size of the virtual image 450, a positionalrelationship among the projection port 301, the combiner 400, and thevirtual image 450, and the like.

Whether to shape the shape of the reflection surface 410 to have ahyperboloid surface, a paraboloid surface, or an ellipsoid surface canbe determined by a decentering relationship between the reference axis420 of the biconic surface forming the reflection surface 410 and thevisual-line direction 340. For example, if the reference axis 420 isdecentered in the x direction with respect to the visual-line direction340, a possible option is to shape the first curve 431, which is theshape in the y direction, to be an ellipsoid line close to a parabola,and shape the second curve 432, which is the shape in the x direction,to be a hyperbolic curve close to a parabola. Also, if the referenceaxis 420 is decentered in both the x direction and the y direction withrespect to the visual-line direction 340, a possible option is to shapethe first curve 431 and the second curve 432 to have a hyperbolic curveclose to a parabola. Various coefficients and constants for defining acurved surface are desirably determined experimentally.

For the combiner 400, an explanation has been given regarding aconfiguration where the vertex position 422 of the biconic surfaceforming the reflection surface 410 is included. However, a vertexposition does not need to be included for a combiner. For example, if itis necessary to use a combiner whose size is smaller than a designedsize due to a spatial restriction or the like in a vehicle, only a partof the reflection surface 410 shown in FIG. 14 that is close to theprojection position 322 may be cut out.

[Rotation and Detachment of Combiner and Projection Unit]

FIGS. 22, 23, and 24 are diagrams for explaining a case where the headup display 10 is attached at an attachment position corresponding to aright steering wheel vehicle and a case where the head up display 10 isattached at an attachment position corresponding to a left steeringwheel vehicle. FIG. 22 shows a state where the projection unit 300 andthe combiner 400 are removed from the optical unit main body 210 in thehead up display 10 attached to a right steering wheel vehicle. In thehead up display 10 attached to a right steering wheel vehicle, theoptical unit main body 210 and the combiner 400 are placed on the rightside, which is a driver's side of the rear-view mirror 600, viewed fromthe driver. The substrate housing portion 100 has the first attachmentsurface 115 and the second attachment surface 117 opposite to the firstattachment surface 115 and is attached to the rear-view mirror 600 in adirection such that the first attachment surface 115 is in contact withthe attachment member 500 (not shown) in FIG. 22. Also, the optical unitmain body 210 has the first main body surface 221 on the same side asthe first attachment surface 115 of the substrate housing portion 100. Asurface of the optical unit main body 210 opposite to the first mainbody surface 221 is the second main body surface 222.

The head up display 10 shown in FIG. 22 is attached to the rear-viewmirror 600 in an arrangement state where the first attachment surface115 of the substrate housing portion 100 and the first main body surface221 of the optical unit main body 210 are directed downward and theprojection port 301 of the projection unit 300 and a lower end 404 ofthe combiner 400 are on the side of the first main body surface 221.Therefore, the projection axis 320 is on the side of the first main bodysurface 221 (see FIG. 1).

FIG. 23 illustrates the head up display 10 attached to a left steeringwheel vehicle. As shown in this figure, when the head up display 10 isattached to a left steering wheel vehicle, the head up display 10 isattached to the rear-view mirror 600 in a direction such that the secondattachment surface 117 is in contact with the attachment member 500 (notshown) while the second attachment surface 117 of the substrate housingportion 100 is directed downward. In this case, the optical unit mainbody 210 and the combiner 400 are placed on the left side, which is adriver's side of the rear-view mirror 600, viewed from the driver.

FIG. 24 is a diagram illustrating the head up display 10 attached to aleft steering wheel vehicle. The head up display 10 is attached to therear-view mirror 600 in an arrangement state where the second attachmentsurface 117 of the substrate housing portion 100 and the second mainbody surface 222 of the optical unit main body 210 are on the samedownside and the projection port 301 of the projection unit 300 and thelower end 404 of the combiner 400 are on the side of the second mainbody surface 222.

As shown in FIG. 22 and FIG. 24, the projection unit 300 and thecombiner 400 can be placed with respect to the optical unit main body210 in either state where the projection port 301 and the lower end 404are located on the first main body surface 221 of the optical unit mainbody 210 or where the projection port 301 and the lower end 404 arelocated on the second main body surface 222 of the optical unit mainbody 210. Also, as shown in FIG. 22 and FIG. 23, it is also possible toremove the projection unit 300 and the combiner 400 from the opticalunit main body 210 and change the respective directions of attachment.It is also possible to connect the projection unit 300 and the combiner400 with the optical unit main body 210 by a rotating member and changethe respective directions of attachment via the rotating member(diagrammatic representation is omitted). In other words, in the head updisplay 10, the projection unit 300 and the combiner 400 can be attachedwhile the respective directions of attachment are changed with respectto the optical unit main body 210. By changing the directions ofattachment, the placement of the projection port 301 emitting imagedisplay light projected on the combiner 400 from the projection unit 300and the projection axis 320 related to the projection direction of theimage display light can be on the side of the first main body surface221 or on the side of the second main body surface 222.

As shown in FIG. 24, even when the second attachment surface 117 is onthe downside, the projection unit 300 can be placed in a state where theprojection port 301 of the projection unit 300 is on the side of thesecond main body surface 222 of the optical unit main body 210, andimage display light is thus projected downward from the optical unitmain body 210. Therefore, the projection axis 320 is on the side of thesecond main body surface 222.

As described above, the projection unit 300 and the combiner 400 can beplaced with respect to the optical unit main body 210 in either statewhere the projection port 301 and the lower end 404 are located on thefirst main body surface 221 of the optical unit main body 210 or wherethe projection port 301 and the lower end 404 are located on the secondmain body surface 222 of the optical unit main body 210. In other words,the projection unit 300 and the combiner 400 can be attached at aposition where the projection port 301 of the projection unit 300 andthe lower end 404 of the combiner 400 are changed 180 degrees withrespect to either one of the surfaces (the first main body surface 221or the second main body surface 222) of the optical unit main body 210.The respective positions of attachment of the projection unit 300 andthe combiner 400 with respect to the optical unit main body 210 can bechanged, and the respective positions of attachment of the projectionunit 300 and the combiner 400 with respect to the first attachmentsurface 115 (or the second attachment surface 117) of the substratehousing portion 100 can be changed.

If the projection unit 300 and the combiner 400 are attached while therespective positions of attachment are changed 180 degrees with respectto the optical unit main body 210, the direction of an image (virtualimage) that is visually recognized on the combiner 400 may change 180degrees compared to the direction before the change of the attachment.In the head up display 10, the circuit substrate 111 outputs an imagesignal in which the direction of an image is changed from the directionbefore the change of the attachment, by detection of the respectiveattachment positions and directions of the projection unit 300 and thecombiner 400 or by setting made by the driver via an operation unit suchas a remote controller or the like.

For example, in the head up display 10 attached as shown in FIG. 22, bychanging the direction of an image output at an attachment positionwhere the projection port 301 of the projection unit 300 is on the sideof the first main body surface 221 and the direction of an image outputat an attachment position where the projection port 301 of theprojection unit 300 is on the side of the second main body surface 222by 180 degrees from each other, an image of the same direction can bevisually recognized even when the attachment position of the projectionunit 300 with respect to the optical unit main body 210 is changed 180degrees.

With this, the image display element 240 changes the direction of animage (upward and downward and leftward and rightward by 180 degrees orthe like) according to the attachment position of the projection unit300 so as to output the image, and the driver can thus visuallyrecognize the image (virtual image) even when the attachment position ischanged.

Also, even when the head up display 10 is attached to a left steeringwheel vehicle, the rotation surface of the hinge 113 is located at aposition where the rotation surface does not cross the rear-view mirror600 in the same way as in the case shown in FIG. 13. Therefore, theoptical unit 200 and the combiner 400 can be rotated in an integralmanner without coming into contact with the rear-view mirror 600 whilethe substrate housing portion 100 is fixed to the rear-view mirror 600.

[Rear-View Mirror Attachment Member]

A detailed description is now given regarding the attachment member 500for attaching the head up display 10 to the rear-view mirror 600. FIG.25 illustrates the attachment member 500 for attaching the head updisplay 10 to the rear-view mirror 600. As shown in the figure, theattachment member 500 has a pair of holding portions 590 fixed to therear-view mirror 600 in such a manner that the holding portions 590 holdthe rear-view mirror 600, and an attachment plate 581 for attaching thepair of holding portions 590 and the substrate housing portion 100. Theholding portions 590 has two lower side holding mechanism portions 591having a hook portion that is slidable in forward and backwarddirections in order to hold a lower end portion of the rear-view mirror600, two upper side holding mechanism portions 592 having a hook portionthat is slidable in the forward and backward directions in order to holdan upper end portion of the rear-view mirror 600, a height adjustingportion 593 that is slidable in the upward and downward directions inorder to hold the rear-view mirror 600 in the upward and downwarddirections from behind, and a position adjustment groove 594, which is along hole for performing position adjustment on the holding portions 590of the attachment plate 581, on the upper surface on which theattachment plate 581 is to be placed. The attachment plate 581 is placedon the respective upper surfaces of the pair of the holding portions 590across the upper surfaces and is attached while a pair of projections584 is engaged with the position adjustment groove 594.

FIG. 26 is a trihedral figure of the attachment plate 581 of theattachment member 500 shown in FIG. 25. As shown in this figure, theattachment plate 581 is formed of an approximately rectangularplate-like member as a whole, and a flat surface that is an attachmentsurface is provided with circular arc holes 582, which are a pair ofcircular-arc shaped holes of different directions, central holes 583,which are a pair of holes formed at respective central positions ofcircles on which the respective circular arcs of the circular arc holes582 are based, and the projections 584 on the back side for allowing theattachment plate 581 to be slidable in the longitudinal direction of theposition adjustment groove 594 by attaching the attachment plate 581 tothe holding portions 590 such that the projections 584 become engagedwith the position adjustment groove 594 formed on the holding portions590.

The central holes 583 are provided in the center of a width direction,which is a direction that is perpendicular to a straight line thatconnects the pair of projections of the attachment plate 581. On theother hand, the pair of projections 584 are not attached in the centerof the width direction described previously but are placed at a positionthat is apart from the center by a certain distance (offset D) in thewidth direction. With this, a sliding range can be changed to be widelydifferent in a first state where the attachment plate 581 is attachedsuch that the projections 584 are located closer to the height adjustingportion 593 than to the respective central holes 583 and a second statewhere the attachment plate 581 is used while switching two ends thereoflocated in the width direction with each other by rotating theattachment plate 581 by 180 degrees using a direction perpendicular tothe surface of the attachment plate 581 as an axis from the first statewhile the pair of projections 584 are facing downward, and an adjustablerange of the position of the substrate housing portion 100 can thus beincreased. Note that the second state is a state where the attachmentplate 581 is attached such that the projections 584 are located fartheraway from the height adjusting portion 593 than from the respectivecentral holes 583. Since a distance between a rear-view mirror 600 and awindshield (a front windshield) of a vehicle varies depending on thetype of the vehicle, by arranging a pair of projections 584 away fromthe center by the offset D, the degree of freedom of a position in theforward and backward directions at which a head up display 10 is fixedwith respect to the rear-view mirror 600 is increased, and the head updisplay 10 can thus be installed in various vehicles. Also, by providinga plurality of holding portions 590 (a pair in the case of the presentembodiment), the head up display 10 can be installed in even morevariety of vehicles.

Regarding a distance between the pair of holding portions 590, the pairof holding portions 590 can be arranged such that a distance between thetwo position adjustment grooves 594 is the same as a distance betweenthe two projections 584 of the attachment plate 581. Alternatively, thepair of holding portions 590 can be arranged such that the distancebetween the two position adjustment grooves 594 becomes shorter than thedistance between the two projections 584. Since the distance between thepair of projections 584 does not change, the attachment plate 581 isconsequently attached in an oblique manner with this arrangement, andthe attachment plate 581 can thus be attached with an angle changed withrespect to the longitudinal direction of the position adjustment grooves594. In other words, the attachment plate 581 and the substrate housingportion 100 can be attached at an angle by rotating the attachment plate581 and the substrate housing portion 100 along a plane on theattachment plate 581. As described, by providing a plurality of holdingportions 590 (a pair in the case of the present embodiment) andadjusting respective distances between the plurality of holding portions590, even more variety of positions for attachment are possible.

When attaching the substrate housing portion 100, a surface of theattachment plate 581 (a surface on which the projections 584 are notprovided) and the first attachment surface or the second attachmentsurface of the substrate housing portion 100 are arranged overlappingwith each other, and setscrews 118 (fixing members) are inserted througha circular arc hole 582 and a central hole 583 located in the center ofa circular arc of the circular arc hole 582 so as to fix the substratehousing portion 100 by screwing. At the time of screwing, the substratehousing portion 100 is movable around the central hole 583 on thesurface of the attachment plate 581, and the direction of the substratehousing portion 100 obtained while a normal to the surface of theattachment plate 581 is used as an rotation axis is adjusted. Since thesubstrate housing portion 100, the optical unit 200, and the combiner400 are rotated in an integral manner around the central hole 583 atthis time, the driver can adjust an attachment angle, that is obtainedwhile the normal to the surface of the attachment plate 581 is used asthe rotation axis, to the position where the driver can visuallyrecognize an image (virtual image) displayed on the combiner 400. Thecentral angle of the circular arc of the circular arc hole 582 is set tobe in a range that is sufficient for the driver to adjust the attachmentangle to the position where the driver can visually recognize an image(virtual image) displayed on the combiner 400. Also, the central angleof the circular arc of the circular arc hole 582 is more preferably setto be in a range where the combiner 400 does not come in contact withthe windshield.

Given that the center direction of the circular arc of the circular archole 582 is referred to as an inner side and that the direction oppositeto the center direction of the circular arc is referred to as an outerside, the pair of the circular arc holes 582 are arranged in such amanner that the respective inner sides thereof face to each other in thepresent embodiment. However, depending on a position where the substratehousing portion 100 is fastened by a setscrew, the pair of the circulararc holes 582 may be arranged in such a manner that the respective outersides thereof face to each other.

FIG. 27 illustrates the head up display 10 attached to the rear-viewmirror 600. The holding portions 590 of the attachment member 500 eachhold the upper end and the lower end of the rear-view mirror 600 fromthe back surface (surface with no mirror in this case) of the rear-viewmirror 600 at two parts, and the attachment plate 581 is attached suchthat the position thereof in the longitudinal direction of the positionadjustment grooves 594, which is mainly a direction perpendicular to themirror surface of the rear-view mirror 600, by the projections 584 beingengaged with the respective position adjustment grooves 594 formed onthe upper side holding mechanism portions 592 of the respective holdingportions 590. Also, the attachment plate 581 is fixed such that an anglethereof obtained while the normal to the surface of the attachment plate581 is used as a rotation axis is adjustable.

Subsequently, an explanation is given using FIG. 27 regarding arelationship between the position of the rear-view mirror 600 and theposition of the combiner 400. The explanation is given on the assumptionthat the longitudinal direction of the rear-view mirror 600 is parallelto a horizontal plane and that the mirror surface is perpendicular tothe horizontal plane. Also, a line that passes through the center of therear-view mirror 600 in the upward and downward directions and that isparallel to the transverse direction of the rear-view mirror 600 isreferred to as a rear-view mirror center line 605. Also, a line thatpasses through the center of the combiner 400 in the upward and downwarddirections and that is parallel to the transverse direction of thecombiner 400 is referred to as a combiner center line 403. In thepresent embodiment, the observation angle of the combiner 400 isadjustable, and the relative height of the combiner 400 with respect tothe height of the rear-view mirror 600 changes with the adjustment ofthe observation angle of the combiner 400. The relative heights of thecombiner 400 and the rear-view mirror 600 can be also said to be adifference between the height of the combiner center line 403 and theheight of the rear-view mirror center line 605. For example, if thecombiner center line 403 is located at a position higher than therear-view mirror center line 605, it can be considered that the combiner400 is located at a position relatively higher than the rear-view mirror600. Also, the position condition of the combiner 400 explained in thefollowing is preferably satisfied at all positions of the combiner 400in a usage state (a state where an image projected can be visuallyrecognized by the user). In other words, although the position conditionis preferably satisfied at all possible observation angles of thecombiner 400, a sufficient effect can be achieved at least as long asthe position condition is satisfied when the combiner 400 has an averageheight of the possible relative height of the combiner 400 with respectto the height of the rear-view mirror 600. For example, if the relativeheight of the combiner 400 with respect to the height of the rear-viewmirror 600 is adjustable from a position where the combiner center line403 is higher than the rear-view mirror center line 605 by 5 cm to aposition where the combiner center line 403 is lower than the rear-viewmirror center line 605 by 5 cm, the position condition needs to besatisfied when the combiner center line 403 and the rear-view mirrorcenter line 605 have the same height. Also, in a case of a configurationwhere the relative height of the combiner 400 with respect to the heightof the rear-view mirror 600 is fixed by screwing or the like such thatthe relative height cannot be adjusted, in other words, in a case of aconfiguration where the relative height of the combiner 400 with respectto the height of the rear-view mirror 600 is fixed (such that therelative height is uniquely determined) with the attachment of the headup display 10 to the rear-view mirror 600 of a vehicle, the positioncondition of the combiner 400 explained in the following needs to besatisfied at the position where the relative height is fixed. As shownin FIG. 27, the rear-view mirror 600 has a length L in the transversedirection (the longitudinal direction) and a height H in the upward anddownward directions.

First, an explanation is given regarding a preferred position conditionof the combiner 400. In the present embodiment, an upper end 402 of thecombiner 400 in the usage state is located higher than the rear-viewmirror center line 605 of the rear-view mirror 600, and a lower end 404of the combiner 400 is located lower than the rear-view mirror centerline 605 of the rear-view mirror 600. By attaching the head up display10 to the rear-view mirror 600 and by achieving an attachment structurewhere the combiner 400 is placed at such a position, the head up display10 can be installed at an optimal position with a small displacement ofa viewpoint at the time of viewing a display image.

Further, a configuration may be employed that allows the combiner centerline 403 of the combiner 400 in the usage state has almost the sameheight as the rear-view mirror center line 605. By attaching the head updisplay 10 to the rear-view mirror 600 and by achieving an attachmentstructure where the combiner 400 is placed at such a position, the headup display 10 can be installed at an optimal position with an evensmaller displacement of a viewpoint at the time of viewing a displayimage.

Also, regarding a case where the height of the combiner 400 in theupward and downward directions is larger than the height H of therear-view mirror 600 in the upward and downward directions, aconfiguration may be employed where the upper end 402 of the combiner400 in the usage state is located higher than the upper end 604 of therear-view mirror 600 and where the lower end 404 of the combiner 400 islocated lower than the lower end 606 of the rear-view mirror 600. Byattaching the head up display 10 to the rear-view mirror 600 and byachieving an attachment structure where the combiner 400 is placed atsuch a position, the head up display 10 can be installed at an optimalposition with an even smaller displacement of a viewpoint at the time ofviewing a display image.

Such a position shown in the present embodiment is optimal. However, atleast as long as the upper end 402 of the combiner 400 in the usagestate is located higher than the lower end 606 of the rear-view mirror600 or the lower end 404 of the combiner 400 is located lower than theupper end 604 of the rear-view mirror 600, the head up display 10 can beinstalled at a preferred position with a small displacement of aviewpoint at the time of viewing a display image. In the presentembodiment, a state where the combiner 400 is at the lateral side of therear-view mirror 600 needs to be a state where the position of thecombiner 400 in the transverse direction is a position that allows adisplay image to be visually recognized from a seat of the vehicle whilesatisfying a condition for exerting this above-described effect. Inother words, it is only necessary that a display image projected on thecombiner 400 is not blocked by the rear-view mirror 600.

In addition to the above-described position condition, the position ofthe combiner 400 in the transverse direction is more preferably arrangedin a range of up to the length L of the rear-view mirror 600 from an endin the transverse direction (lateral end) of the rear-view mirror 600since the rear-view mirror 600 and the combiner 400 are not too far awayfrom each other with a small displacement of a viewpoint.

FIG. 28 is a cross-sectional view of a portion including a setscrew 118when the first attachment surface 115 of the substrate housing portion100 is attached such that the first attachment surface 115 is in contactwith the attachment plate 581. FIG. 29 is a cross-sectional view of aportion including a setscrew 118 when the second attachment surface 117of the substrate housing portion 100 is attached such that the secondattachment surface 117 is in contact with the attachment plate 581. Ingeneral, a space between the upper side of the rear-view mirror 600 anda ceiling is extremely small. Thus, a setscrew 118 is tighten only frombelow for a case where the first attachment surface 115 is in contactwith the attachment plate 581 and for a case where the second attachmentsurface 117 is in contact with the attachment plate 581. Also, since thesubstrate housing portion 100 is designed to be as thin as possible, thecircuit substrate 111 has a through hole at a fixing position by thesetscrew 118, allowing for fixation by a longer screw. An insert nut116, which is a fixing member engagement unit that extends to the secondattachment surface 117, is formed on the first attachment surface 115,and a through hole is formed at a corresponding position on the secondattachment surface 117. Thus, the setscrew 118 is fixed in engagementwith the same insert nut 116 for a case where the first attachmentsurface 115 is in contact with the attachment plate 581 and for a casewhere the second attachment surface 117 is in contact with theattachment plate 581. Therefore, the substrate housing portion 100 canbe installed even in a small area between the rear-view mirror 600 ofthe vehicle and the ceiling. Therefore, the position and the angle canbe adjusted in a space-saving manner in the head up display 10 accordingto the present embodiment.

FIG. 30 illustrates an attachment plate 571, which is an exemplaryvariation of the attachment plate 581. The attachment plate 571 has apair of linear straight-line hole portions 572 that extend in onedirection, which are used when attaching the substrate housing portion100. Setscrews 118 are inserted through both of the straight-line holeportions 572 even when an attachment surface of the attachment plate 571faces an attachment surface of either of the first attachment surface115 and the second attachment surface 117 of the substrate housingportion 100. In the attachment plate 571, by attaching the substratehousing portion 100 while changing respective attachment positions inthe longitudinal direction of both of the pair of the straight-line holeportions 572, the position of the substrate housing portion 100 in thelongitudinal direction can be adjusted. In this case, each hole of thestraight-line hole portions 572 is formed to have a width that issufficiently larger than the screw diameter of a setscrew 118. Withthis, the direction of the substrate housing portion 100 with the normalto the surface of the attachment plate 581 of the substrate housingportion 100 being used as an rotation axis can be adjusted by changingone of the attachment positions in the longitudinal direction of thepair of the straight-line hole portions 572. The respective lengths andwidths of the straight-line hole portions 572 are determined in a rangewhere the combiner 400 does not come in contact with the windshield.

As described, although a pair of long holes that are circular-arc shapedare used in the case of above-described attachment plate 581, thedirection of the substrate housing portion 100 can be freely adjustedeven when a pair of linear long holes are used as in the case of theattachment plate 571, which is an exemplary variation of the attachmentplate 581. In modes explained using FIGS. 25 through 30, examples areshown where the substrate housing portion 100 and the optical unit 200are formed separately. However, the same can apply even when thesubstrate housing portion 100 and the optical unit 200 are integrallyformed. In the modes explained using FIGS. 25 through 30, two positionadjustment grooves 594 are used. Alternatively, one or more grooveshaving a function of a position adjustment may be used.

[Combiner Storage]

FIGS. 31 and 32 are a lateral view and a front view showing a statewhere the combiner 400 is placed at a storage position by a storagehinge 472, respectively. As shown in FIGS. 31 and 32, the combiner 400is rotated by the storage hinge 472, which is a rotating unit of thecombiner 400, for storage so as to face a housing surface of the opticalunit 200, i.e., a housing surface of the optical unit main body 210 suchthat, for example, the combiner 400 overlaps the housing surface. Inthis case, the projection unit 300 is located on an opposite side from aside on which the combiner 400 is attached across the housing surface,and a length from the rotation center of the storage hinge 472 to thelower end 404, which is an end of the combiner 400 that is the farthestfrom the rotation center, is shorter than the length of the optical unitmain body 210. The lower end 404 is located more to the side of thestorage hinge 472 than the projection unit 300. Also, the height of theoptical unit main body 210 from the housing surface is shorter than theheight of the projection unit 300 from the housing surface. Therefore,when the head up display 10 is not being used, by storing the combiner400 by the storage hinge 472, the combiner 400 can be placed at aposition where the combiner 400 does not give a feeling of oppression tothe driver compared to when the combiner 400 is being used (a positionwhere the combiner 400 does not come into the driver's field of visioncompared to when the combiner 400 is being used). Also, by storing thecombiner 400 by rotating the combiner 400 using the storage hinge 472,sunlight can be prevented by the ceiling of the vehicle and the opticalunit main body 210, and the deterioration of the combiner 400 can thusbe prevented. Further, the storage hinge 472 stops at an angle formedwhen the combiner 400 is used. Thus, when start using the combiner 400again after storing the combiner 400 by rotating the combiner 400 by thestorage hinge 472, the driver can start using the combiner 400 withoutadjusting the position again. Transparent rubber 406 may be attached ata corner portion on the side of the lower end 404 of the combiner 400.Even when the combiner 400 is stored by the storage hinge 472 by pickingthe rubber 406, adhesion of dirt or the like to the combiner 400 can beprevented. Being transparent, the rubber 406 hardly blocks the field ofview of the driver.

The attachment is made on the back side of the rear-view mirror 600.Alternatively, the attachment may be made to a post of the rear-viewmirror 600 or may be made on the front side, which is the mirror surface602. In this case, an alternative mirror may be placed on a surface ofthe display device for vehicle at a position corresponding to the mirrorsurface 602.

Also, in the above-described embodiment, as long as the rear-view mirror600 is a mirror that can be used to check behind the vehicle in thevehicle, the position or the like of the mirror inside the vehicle isnot limited. Also, the head up display 10 is attached to the rear-viewmirror 600. Alternatively, the head up display 10 may be placed on thedashboard for use. A display device for vehicle may be realized byplacing a display device such as a liquid crystal display device or anorganic EL display device at the position of the combiner 400.

[Types of Intermediate Image Screen]

As described above, the intermediate image screen 360 images an imagegenerated by the image display element 240 so as to generate a realimage. In this case, methods for realizing the intermediate image screen360 include at least two methods, “transmission-type” and“reflection-type” methods.

In a “transmission-type” intermediate image screen 360, video light thathas entered one surface of the screen passes through the screen and isemitted from the other surface of the screen. On the other hand, in a“reflection-type” intermediate image screen 360, video light that hasentered one surface of the screen is reflected near the other side ofthe screen and is emitted again from the surface on which the videolight has entered. In the following, a “transmission-type” intermediateimage screen is stated as a transmission-type intermediate image screen361, a “reflection-type” intermediate image screen is stated as areflection-type intermediate image screen 362, and both of the screensare collectively referred to as intermediate image screens 360 when thescreens are not particularly differentiated in the subjectspecification. In the following, an explanation is given of atransmission-type intermediate image screen 361 in reference to figures.

[Transmission-Type Intermediate Image Screen]

In a transmission-type screen used in a conventional display device suchas a projector used indoors (hereinafter, referred to as a“transmission-type screen for regular uses”), which is not a displaydevice for vehicle, a gain is low making the screen dark, and a viewingangle is wide. Therefore, a transmission-type screen for regular uses isnot adequate for use in a head up display as a display device forvehicle. On the other hand, when a diffusion sheet with a haze value(cloudiness) that is lower than that of a transmission-type screen forregular uses is used, a hot spot of a light source becomes too bright,and brightness distribution becomes too large. Thus, a video imagebecomes hard to see.

In order to overcome these problems, a transmission-type intermediateimage screen has been developed that projects a video image on anappropriate transmission-type high-gain diffusion film or diffusionplate with an light distribution. However, a transmission-typeintermediate image screen for a head up display is expected to show, ona combiner 400 or a windshield, a real image that is imaged on thescreen so as to allow the user, who is the driver, to recognize anenlarged virtual image of the real image. Therefore, a transmission-typeintermediate image screen for a head up display is required to have anextremely small screen size and a high resolution compared to atransmission-type screen for regular uses.

FIGS. 33A-33B are cross-sectional views schematically illustrating across-sectional surface of a transmission-type intermediate image screen361 according to the embodiment. More specifically, FIG. 33A illustratesa cross-sectional view of a transmission-type intermediate image screen361 in which a diffusion layer is formed by applying bead diffusionmaterials 364 on a plastic base 363, and FIG. 33B illustrates across-sectional view of a transmission-type intermediate image screen361 in which a diffusion layer is formed including bead diffusionmaterials 364 in an acrylic base material 365.

In examples of a transmission-type intermediate image screen 361 shownin FIGS. 33A and 33B, a haze value is 84 to 90 percent in both examples,and highly transparent beads for optical use having a diameter of 10micrometer or less are used as diffusion materials. A transmission lightdistribution angle formed when parallel light is made incident on thesetransmission-type intermediate image screens 361 is a luminous intensityhalf-value angle of ±7.5 to 10 degrees. This transmission lightdistribution angle is a value measured by a variable-angle photometerGC5000L manufactured by Nippon Denshoku Industries Co., Ltd.

As shown in FIG. 33A, when applying the bead diffusion materials 364 onthe plastic base 363, the bead diffusion materials 364 are fixed by apredetermined binder. However, if the thickness of the diffusion layeris approximately 50 micrometers or more, it is no longer necessary toreinforce the diffusion layer by the plastic shown in FIG. 33A. Thethickness of the diffusion layer can be changed by including the beaddiffusion materials 364 in the acrylic base material 365 as shown inFIG. 33B, when making the thickness of the diffusion layer to beapproximately 50 micrometers or more.

As described above, the head up display 10 according to the embodimentpresents a real image imaged by the transmission-type intermediate imagescreen 361 to the user, who is the driver, via the combiner 400. Thehead up display 10 according to the embodiment is based on theassumption that the user observes a video image of a size of about 10inches approximately 1.7 to 2 meters ahead via the combiner 400. Underthis condition, resolution that allows the user having visual acuity of2.0 to recognize a presented virtual image when the user views thepresented virtual image is about 40 to 50 micrometers on thetransmission-type intermediate image screen 361.

In general, a user having visual acuity of 2.0 is considered to havesufficient visual acuity, and most users are considered to have visualacuity of less than 2.0. Therefore, if the resolution of a real imageformed on the transmission-type intermediate image screen 361 is about50 micrometers or greater under the above condition, it can beconsidered that a video image having resolution that is sufficient forthe user can be provided.

Also, the head up display 10 according to the embodiment is designedsuch that a viewing angle of a space where a virtual image presented bythe combiner 400 is visually recognizable is ensured to have at leastabout ±10 degrees. Thus, as described above, a transmission-typeintermediate image screen 361 is employed that has a transmission lightdistribution angle, which is a luminous intensity half-value angle of±7.5 to 10 degrees.

It should be understood that the above specific numerical values arejust examples, and a person skilled in the art should easily appreciatethat these values can be freely changed based on usage scenes of thehead up display 10.

FIG. 34 is a diagram schematically illustrating a relationship among thethickness T of a diffusion layer, a half-width at half-maximum angle Aof a transmission light distribution angle, and the resolution R of avideo image formed on the transmission-type intermediate image screen361. FIG. 34 illustrates that light that is incident on a point U on asurface 366 of the diffusion layer is diffused in the diffusion layer atthe transmission light distribution angle of a luminous intensityhalf-width at half-maximum angle A. Light that is incident on the singlepoint U on the surface 366 of the diffusion layer is diffused and spreadout between a point V and a point W while maintaining light intensitydistribution such as the one shown in FIG. 34 on a surface 367 on theside opposite to an incident surface of the diffusion layer. When adistance from the point V to the point W is set to be R, light that isincident on a single point on the surface 366 of the diffusion layer isspread out while maintaining distribution in a circle of a diameter of Rwith light intensity of up to 0.5. As the size of this distance Rbecomes smaller, there is less overlapping of image display light. Thus,detailed expression of a video image can be possible on the surface 367on the side opposite to the incident surface of the diffusion layer. Inthis sense, the inventors of the subject application have found that theresolution on the surface 367 on the side opposite to the incidentsurface of the diffusion layer can be approximated by the distance Rfrom the point V at which image display light having light intensity of0.5, where the value of luminous intensity is half at the transmissionlight distribution angle, overlaps with neighboring image display lighthaving light intensity of 0.5 to the point W at which image displaylight having light intensity of 0.5 overlaps with neighboring imagedisplay light having light intensity of 0.5 in the same way.

In FIG. 34, a relationship among the thickness T of the diffusion layer,the half-width at half-maximum angle A of the transmission lightdistribution angle, and the distance R from the point V to the point Wcan be expressed by the following Expression (2):T*tan(A)*2=R  (2)

As is obvious from Expression (2), the resolution R is proportional tothe thickness T of the diffusion layer. Therefore, if the resolution Rbeing a target value in the designing and the half-width at half-maximumangle A of the transmission light distribution angle are determined, acondition to be satisfied by the thickness T of the diffusion layer canbe expressed by the following Expression (3):0<T≦R/(2*tan(A))  (3)

In this case, a condition “0<T” is a condition for the diffusion layerto exist, and a condition “T≦R/(2*tan (A))” is a condition for ensuringthe resolution R, which is the target value in the designing. The“target value” is a lower limit value of resolution which a video imageon the transmission-type intermediate image screen 361 needs to have inorder to achieve resolution that needs to be ensured by a virtual imagepresented by the head up display 10 according to the embodiment. Sincethe “target value” is the lower limit value of the targeted resolution,achieving resolution that is higher than the “target value” is not aproblem but is rather preferred. A specific value of the target valueneeds to be determined in consideration of various parameters such as adistance between a virtual image and a user expected by the head updisplay 10, the size of the virtual image to be presented, and visualacuity of the user. An example of the specific value of the target valueis about 40 to 50 micrometers as described above.

FIG. 35 is a diagram illustrating, in a table format, results ofresearching influence of the thickness T of the diffusion layer on theresolution of a real image formed on a surface of the transmission-typeintermediate image screen 361 by changing the thickness T of thediffusion layer, and calculated values of the resolution R that areobtained using Expression (2). As shown in FIG. 35, as the value of thethickness T of the diffusion layer increases, the resolution of thetransmission-type intermediate image screen 361 decreases. Also, it canbe understood that the calculated values of the resolution R obtainedusing Expression (2) are close to the resolution R of the real image onthe transmission-type intermediate image screen 361 obtained byexperiments.

FIG. 36 is a graph illustrating a relationship between the thickness Tof the diffusion layer and the resolution R of the real image formed onthe surface of the transmission-type intermediate image screen 361 and arelationship between the thickness T of the diffusion layer and thecalculated values of the resolution R that are obtained using Expression(2). As described above, in the head up display 10 according to theembodiment, if the resolution R of a real image formed on thetransmission-type intermediate image screen 361 is about 50 micrometers,a video image having sufficient resolution can be provided to the user.As shown in FIG. 36, a condition that needs to be satisfied by thethickness T of the diffusion layer in order for the resolution R of thereal image formed on the surface of the transmission-type intermediateimage screen 361 to be 50 micrometers or less is that T is 140micrometers or less. As shown in comparative examples 1-3 in FIG. 35, ithas been confirmed by experiments that when the thickness T of thediffusion layer becomes thicker than 125 micrometers, the resolution Rof the real image formed on the surface of the transmission-typeintermediate image screen 361 becomes 50 micrometers or more.

Summarizing the above, when presenting to a user a video image of a sizeof about 10 inches and a viewing angle of 10 degrees approximately 1.7to 2 meters ahead via the combiner 400 using the head up display 10according to the embodiment, the thickness T of the diffusion layer inthe transmission-type intermediate image screen 361 is preferably set tobe 125 micrometers or less. By setting the thickness of the diffusionlayer in the transmission-type intermediate image screen 361 to be 125micrometers or less, a video image that has a wide viewing angle, thatis bright without a hot spot, and that has sufficient resolution can beprovided when a user having visual acuity of 2.0 or less views a virtualimage of about 10 inches 1.7 to 2 meters or more ahead.

[Reflection-Type Intermediate Image Screen]

In the above, a case where a transmission-type intermediate image screen361 is used as an intermediate image screen 360 has been explained. Acase where a reflection-type intermediate image screen 362 is used as anintermediate image screen 360 is now explained. For the sake of ease ofexplanation, an explanation is given based on the assumption that anon-dashboard-type head up display 11, which is installed on a dashboardof a car or the like for use, is used as a head up display. However, aperson skilled in the art should easily appreciate that areflection-type intermediate image screen 362 can be also used in a headup display 10 that is designed to be attached to a rear-view mirror 600for use.

FIG. 37 is a perspective view showing the exterior appearance of anon-dashboard-type head up display 11 according to the embodiment. Theon-dashboard-type head up display 11 includes a main body 20 that storesa control substrate and an optical unit, a combiner 400, areflection-type intermediate image screen 362, a heat dissipation unit21 having ventilation holes 22 and 23, and a heat pipe cover 24.

A heat pipe 25 is stored inside the heat pipe cover 24, and the heatpipe 25 transmits heat generated inside the main body 20 to the heatdissipation unit 21. The heat dissipation unit 21 includes a heat sink243 and a cooling fan 26 and discharges heat generated by theon-dashboard-type head up display 11 to the outside.

FIG. 38 is a diagram schematically illustrating a relationship betweenan installation position of the on-dashboard-type head up display 11 andthe position of a virtual image 450 presented to a driver C. In FIG. 38,video light projected from the main body 20 of the on-dashboard-typehead up display 11 installed on a dashboard is imaged and reflected on areflection-type intermediate image screen 362 and projected onto thecombiner 400. For the driver C observing a video image projected ontothe combiner 400, the virtual image 450 is observed as if the virtualimage 450 exists further away in the line of sight with respect to thecombiner 400. The internal configuration of the on-dashboard-type headup display 11 and the operation thereof are the same as in the case ofthe head up display 10 described above. Therefore, explanations that arethe same as those described for the head up display 10 are appropriatelyomitted or simplified in the following.

There are various variations of conventional reflection-type screens forregular uses such as mat type, bead type, pearl type, silver type, orsound screen type reflection-type screens. However, in any of thevariations, a gain is low making the screen dark, and a viewing angle iswide. Thus, none of them are adequate for a head up display. Also,specular reflection caused by a specular surface creates a problem wherea hot spot of the light source 231 becomes too bright for a user andthat a video image therefore becomes hard to see since brightnessdistribution becomes too large.

In order to overcome these problems, a reflection-type screen has beendeveloped that laminates a transmission-type high-gain diffusion layeror diffusion film with an optimal light distribution directly on aplate-like or sheet-like specular reflection surface and that projects avideo image on the surface thereof. However, a reflection-typeintermediate image screen 362 for a head up display is expected to show,on a combiner 400 or a windshield, a real image that is imaged on thescreen so as to allow the user, who is the driver, to observe anenlarged virtual image of the real image. Therefore, a reflection-typeintermediate image screen 362 for a head up display is required to havea small screen size and a high resolution compared to a reflection-typescreen for regular uses.

FIG. 39 is a cross-sectional view schematically illustrating across-sectional surface of a reflection-type intermediate image screen362 according to the embodiment. In the reflection-type intermediateimage screen 362, bead diffusion materials 364, a first film base 370, afirst adhesive layer 371, a reflection film 372 on which a silver screenis deposited, a second film base 373, a second adhesive layer 374, and areinforcement base plate 375 are laminated in order from a lightincident surface.

In FIG. 39, light that is incident into a layer of the bead diffusionmaterials 364 is diffused by the bead diffusion materials 364 reachingthe reflection film 372 and is then reflected by the reflection film 372such that the light reaches the bead diffusion materials 364 again.Therefore, in the reflection-type intermediate image screen 362, it isconsidered that the thickness of a layer obtained by combining the beaddiffusion materials 364 and the first film base 370 has an influence onthe resolution of the screen. Also, the second film base 373 and thereinforcement base plate 375 have a function of facilitating handling bya user by providing strength to the reflection-type intermediate imagescreen 362.

As in the case of the transmission-type intermediate image screen 361shown in FIG. 33, the bead diffusion materials 364 shown in FIG. 39 arehighly transparent beads for optics, and the diameter thereof is 10micrometers or less. The bead diffusion materials 364 are applied on asurface of the first film base 370 in a thickness of 10 to 15micrometers. A reflection light distribution viewing angle formed whenparallel light is made incident on this is a luminous intensityhalf-value angle of ±7.5 to 10 degrees. This reflection lightdistribution angle is a value measured by a variable-angle photometerGC5000L manufactured by Nippon Denshoku Industries Co., Ltd.

FIG. 40 is a diagram schematically illustrating a relationship among adistance L from an incident surface to a reflection surface of imagedisplay light in the diffusion layer in the reflection-type intermediateimage screen, a half-width at half-maximum angle A of a reflection lightdistribution angle, and the resolution R of a video image formed on thereflection-type intermediate image screen 362. FIG. 40 illustrates thatlight that is incident on a point U′ on a surface 376 of the diffusionlayer is diffused at the half-width at half-maximum angle A of thereflection light distribution angle. Light that is incident on thesingle point U′ on the surface 376 of the diffusion layer is diffused atthat point, reflected at a point X on a reflection surface 377, thendiffused again, and emitted from a point V′ and a point W′ on thesurface 376 of the diffusion layer. When a distance from the point V′ tothe point W′ is set to be R, light that is incident on the single pointU′ on the surface 376 of the diffusion layer is reflected at thereflection surface 377 and spread out while maintaining distribution ina circle of a diameter of R with light intensity of up to 0.5. As thesize of this distance R becomes smaller, there is less overlapping ofimage display light. Thus, detailed expression of a video image can bepossible on the surface 376 of the diffusion layer, which also serves asa light incident surface and a light emission surface of the diffusionlayer. In this sense, the inventors of the subject application havefound that the resolution on the surface 376 of the diffusion layer canbe approximated by the distance R from the point V′ at which imagedisplay light having light intensity of 0.5, where the value of luminousintensity is half at the reflection light distribution angle, overlapswith neighboring image display light having light intensity of 0.5 tothe point W′ at which image display light having light intensity of 0.5overlaps with neighboring image display light having light intensity of0.5 in the same way.

In FIG. 40, a relationship among the distance L from an incident surfaceof image display light in the diffusion layer to a reflection surface ofthe image display light that is incident, the half-width at half-maximumangle A of the reflection light distribution angle, and the distance Rfrom the point V′ to the point W′ can be expressed by the followingExpression (4):L*tan(A)*2=R  (4)

As is obvious from Expression (4), the resolution R is proportional tothe distance L from the incident surface to the reflection surface ofthe image display light in the diffusion layer. Therefore, if theresolution R being a target value in the designing and the half-width athalf-maximum angle A of the reflection light distribution angle aredetermined, a condition to be satisfied by the distance L from theincident surface to the reflection surface of the image display light inthe diffusion layer can be expressed by the following Expression (5):0<L≦R/(2*tan(A))  (5)

In this case, a condition “0<L” is a condition for the diffusion layerto exist, and a condition “L≦R/(2*tan (A))” is a condition for ensuringthe resolution R, which is the target value in the designing.

FIG. 41 is a diagram illustrating, in a table format, results ofresearching influence of the distance L to the reflection surface on theresolution of a real image formed on a surface of the reflection-typeintermediate image screen 362 by changing the distance L from theincident surface to the reflection surface of the image display light inthe diffusion layer, and calculated values of the resolution R that areobtained using Expression (4). As shown in FIG. 41, as the value of thedistance L to the reflection surface increases, the resolution of thereflection-type intermediate image screen 362 decreases. Also, it can beunderstood that the calculated values of the resolution R obtained usingExpression (4) are close to the resolution R of the real image on thereflection-type intermediate image screen 362 obtained by experiments.

FIG. 42 is a graph illustrating a relationship between the distance Lfrom the incident surface to the reflection surface of the image displaylight in the diffusion layer and the resolution R of a real image formedon a surface of the reflection-type intermediate image screen 362 and arelationship between the distance L to the reflection surface and thecalculated values of the resolution R that are obtained using Expression(4). As in the case of the head up display 10, in the on-dashboard-typehead up display 11, if the resolution R of a real image formed on thereflection-type intermediate image screen 362 is about 50 micrometers, avideo image having sufficient resolution can be also provided to theuser. As shown in FIG. 42, a condition that needs to be satisfied by thedistance L from the incident surface to the reflection surface of theimage display light in the diffusion layer in order for the resolution Rof the real image formed on the surface of the reflection-typeintermediate image screen 362 to be 50 micrometers or less is that L is140 micrometers or less. As shown in comparative examples 1-3 in FIG.41, it has been confirmed by experiments that when the distance L fromthe incident surface to the reflection surface of the image displaylight in the diffusion layer becomes thicker than 110 micrometers, theresolution R of the real image formed on the surface of thereflection-type intermediate image screen 362 becomes 50 micrometers ormore.

Summarizing the above, when presenting to a user a video image of a sizeof about 10 inches and a viewing angle of ±10 degrees approximately 1.7to 2 meters ahead via the combiner 400 using the on-dashboard-type headup display 11 according to the embodiment, the distance L from theincident surface to the reflection surface of the image display light inthe diffusion layer in the reflection-type intermediate image screen 362is preferably set to be 110 micrometers or less. By setting the distanceL from the incident surface to the reflection surface of the imagedisplay light in the diffusion layer in the reflection-type intermediateimage screen 362 to be 110 micrometers or less, a video image that has awide viewing angle, that is bright without a hot spot, and that hassufficient resolution can be provided when a user having visual acuityof 2.0 or less views a virtual image of about 10 inches 1.7 to 2 metersor more ahead.

[Installation of Intermediate Image Screen]

As described above, an intermediate image screen 360 according to theembodiment has a thickness of approximately 20 micrometers to 200micrometers regardless of the types of “transmission-type” and“reflection-type.” Therefore, since an intermediate image screen alonelacks rigidity, the intermediate image screen 360 is preferably heldusing some sort of holding member during handling such as installation,replacement, or the like. Further, even when an intermediate imagescreen 360 having sufficient rigidity is used, some sort of protectionmember is preferably installed in order to prevent soiling, scratching,or the like of the intermediate image screen 360. An explanation isgiven in the following regarding the holding and protection of thetransmission-type intermediate image screen 361 according to theembodiment.

FIG. 43 is a diagram schematically illustrating an example of athree-layer portion 380 according to the embodiment. The three-layerportion 380 has a three-layer structure including a first plate 380 aand a second plate 380 b with a transmission-type intermediate imagescreen 361 put between the first plate 380 a and the second plate 380 b.

With respect to the transmission-type intermediate image screen 361, thefirst plate 380 a is a front surface plate located on an incidentsurface of image display light and functions as a protection plate thatprotects the transmission-type intermediate image screen 361. The firstplate 380 a is a highly transparent plastic such as acrylic,polycarbonate, or the like, and dust proof, chemical resistant, andscratch resistant features are provided to a surface on which imagedisplay light is incident.

The second plate 380 b is a rear surface plate provided facing the firstplate 380 a while having the transmission-type intermediate image screen361 in between and functions as a protection plate that protects thetransmission-type intermediate image screen 361 just like the frontsurface plate. The second plate 380 b is also a highly transparentplastic such as acrylic, polycarbonate, or the like just like the firstplate 380 a. The second plate 380 b along with the first plate 380 aholds the transmission-type intermediate image screen 361 therebetween,keeping the shape of the transmission-type intermediate image screen 361flat by preventing warpage or undulation of the transmission-typeintermediate image screen 361 so as to reinforce the transmission-typeintermediate image screen 361 such that the holding position does notmove. An example of the size of the first plate 380 a and the secondplate 380 b according to the embodiment is 19.0 mm height, 13.0 mmwidth, and 1 mm thickness. In comparison to the transmission-typeintermediate image screen 361, the first plate 380 a and the secondplate 380 b are thicker and thus function as reinforcing plates thatreinforce the installed condition of the transmission-type intermediateimage screen 361.

As described, the three-layer portion 380 has a multi-layer structurehaving at least three layer portion of the first plate 380 a, thetransmission-type intermediate image screen 361, and the second plate380 b. However, the holding position may move if these layers are merelystacked in a tightly attached manner. Therefore, this multi-layerstructure is preferably fixed by some sort of means. Thus, a possiblemethod is to fix the first plate 380 a and the transmission-typeintermediate image screen 361 or the second plate 380 b and thetransmission-type intermediate image screen 361 putting an adhesivelayer, which includes glue or adhesive, therebetween. However, theinventors of the subject application have found that the use of anadhesive layer for the fixation of the transmission-type intermediateimage screen 361 may cause “yellow discoloration” where the adhesivelayer turns yellow due to changes over time that are caused bytemperature, humidity, and the like and that this may cause degradationin performance as a screen as a result.

Thus, in the three-layer portion 380 according to the embodiment,instead of fixing the three layers: the first plate 380 a; thetransmission-type intermediate image screen 361; and the second plate380 b, by forming an adhesive layer, a means is used that allows thesethree layers to be fixedly held.

FIG. 44 is a diagram schematically illustrating a screen holding unit390 according to the embodiment. A storage space 392 for storing thethree-layer portion 380 is provided in the screen holding unit 390. Thestorage space 392 is a space in which the three-layer portion 380 is setin and held. FIG. 44 illustrates a hollowed-out step created byhollowing out a portion of the screen holding unit 390 as an example ofthe storage space 392. As shown in FIG. 44, the storage space 392 isprovided with an opening that serves as an optical path for imagedisplay light. The screen holding unit 390 is provided with hold-downmembers 394 for fixing the three-layer portion 380 set in the storagespace 392. FIG. 44 illustrates leaf-spring shapedhold-down clips thatare attached using screws 396 as an example of the hold-down members394. FIG. 44 is a diagram illustrating a case where two hold-downmembers 394 are provided on one side of the screen holding unit 390. Thehold-down members 394 need to be provided on the screen holding unit 390such that at least two parts of the three-layer portion 380 are helddown. Also, the number of the hold-down members 394 is not limited totwo. For example, total of four hold-down members 394 may be providedhaving one at each of the four corners of the screen holding unit 390.Further, the hold-down members 394 may be formed to store thethree-layer portion 380 in the storage space 392 while the three-layerportion 380 is held down, as a frame-like hold-down means that hold downthe four sides of the three-layer portion 380 in an integral manner.

FIG. 45 is a top view showing a state where the three-layer portion 380is installed in the screen holding unit 390. The hold-down members 394are freely rotatable using the screws 396 as respective rotation axes.Therefore, when setting the three-layer portion 380 in the storage space392, the three-layer portion 380 can be fixed in the storage space 392by pulling hold-down members 394 back from the storage space 392 andthen rotating the hold-down members 394 to the point where the hold-downmembers 394 come in contact with the first plate 380 a after thethree-layer portion 380 is set in the storage space 392.

FIG. 46 is a diagram explaining a state where the three-layer portion380 is installed in the screen holding unit 390 and is a view that issectioned along a line A-A shown in FIG. 45 in a state where thethree-layer portion 380 is installed. As shown in FIG. 46, thethree-layer portion 380 set in the storage space 392 is fixed whilebeing held down on the screen holding unit 390 by elastic force of thehold-down members 394.

This allows the screen holding unit 390 to easily fix the three-layerportion 380. Also, since the three layers: the first plate 380 a; thetransmission-type intermediate image screen 361; and the second plate380 b, are fixed without using an adhesive layer, changes that arecaused over time can be reduced. Also, since the three layers: the firstplate 380 a; the transmission-type intermediate image screen 361; andthe second plate 380 b, are not bonded, the transmission-typeintermediate image screen 361 can be freely removed from or installed inthe three-layer portion 380. This allows, for example, the replacementof only the transmission-type intermediate image screen 361. Thus, it ispossible to replace a transmission-type intermediate image screen 361whose performance is lowered due to changes over time. Further, aviewing angle, i.e., a transmission-type intermediate image screen 361with the transmission light distribution angle shown in FIG. 34, can beinstalled according to the preference of a user, who is a driver.

FIG. 47 is a diagram illustrating a state where the screen holding unit390 is installed in the projection unit 300 according to the embodimentand is a cross-sectional view of the projection unit 300 on a planeformed by an axis showing the upward direction and an axis showing theforward direction in FIG. 1.

As shown in FIG. 47, the projection unit 300 is provided with aninstallation groove 310 for fixing the screen holding unit 390 bysetting the screen holding unit 390 therein. Therefore, the screenholding unit 390 is detachable in the direction of the projection port301 of the projection unit 300. As described above, since the projectionunit 300 is detachably attached to the optical unit main body 210, onlythe projection unit 300 can be removed so as to remove or install thescreen holding unit 390 even in a state where the head up display 10 isbeing installed at the rear-view mirror 600.

As explained above, according to a head up display 10 according to theembodiment of the present invention, a technology can be provided thatfacilitates the replacement of a screen in the head up display. Also, bynot using an adhesive layer for the fixation of the transmission-typeintermediate image screen 361, degradation in optical performance due toyellow discoloration of an adhesive layer does not occur.

Described above is an explanation of the present invention based on theembodiment. The embodiment is intended to be illustrative only, and itwill be obvious to those skilled in the art that various modificationsto constituting elements and processes could be developed and that suchmodifications are also within the scope of the present invention.

[Exemplary Variations of Intermediate Image Screen]

In the above embodiment of the present invention, an explanation is maderegarding a case where a diffusion layer having a haze value(cloudiness) of 84 to 90 percent when parallel light is made incidentthereon is used in a transmission-type intermediate image screen 361 anda reflection-type intermediate image screen 362. Regarding a diffusionlayer and surface nature of a diffusion sheet, as long as the haze value(cloudiness) thereof is 84 to 90 percent, any kind of diffusion may beemployed such as diffusion of a concavo-convex shape type, diffusion ofan air-bubble type, diffusion of a lens type, diffusion of a reliefhologram pattern, and the like, instead of bead diffusion. Needless tosay, it is necessary for a particle diameter of a diffusion material, alens pitch, a concavo-convex shape pitch, a pattern pitch, and anair-bubble diameter, which are the smallest units for having a diffusionfunction of forming a diffusion layer of the intermediate image screen,to be smaller than the target value R of the resolution of a real imageformed on the intermediate image screen in order to allow for easyanalogy. Furthermore, for the reflection surface of the reflection-typeintermediate image screen 362, a specular surface aluminum film sheetmay be used instead of a specular surface silver film sheet. Also, aslong as a high-reflectivity specular reflection surface is used under adiffusion layer or a diffusion film, the specular reflection surface mayhave a plate-like shape instead of a sheet-like shape.

[Exemplary Variations of Three-Layer Portion]

In the above embodiment of the present invention, an explanation is maderegarding a case where the three layers: the first plate 380 a; thetransmission-type intermediate image screen 361; and the second plate380 b, in the three-layer portion 380 are separate. As long as at leastthe transmission-type intermediate image screen 361 can be removed fromor installed in the three-layer portion 380, all the three layers do notalways need to be separate.

FIG. 48 is a diagram schematically illustrating another example of thethree-layer portion 380 according to the embodiment. In an example shownin FIG. 48, a first plate 380 a and a second plate 380 b are connectedby small hinges 382. Therefore, the first plate 380 a and the secondplate 380 b are freely rotatable by the hinges 382. As described above,the first plate 380 a and the second plate 380 b are small being 19.0 mmheight, 13.0 mm width, and 1 mm thickness. Therefore, in comparison witha case where the two are independently and separately movable, there isan advantage of facilitating a work of, e.g., replacing thetransmission-type intermediate image screen 361 when the movement of thetwo are limited as shown in FIG. 48.

Although not shown in the figure, as yet another example of the shape ofthe three-layer portion 380 according to the embodiment, the three-layerportion 380 may be shaped such that three sides of the first plate 380 aand three sides of the second plate 380 b that correspond to the threesides of the first plate 380 a are connected and that the respectiveremaining sides are open allowing the transmission-type intermediateimage screen 361 to be put in and out. As yet another example of theshape of the three-layer portion 380 according to the embodiment, thethree-layer portion 380 may be shaped in a, so to speak, clear foldershape such that two sides of the first plate 380 a and two sides of thesecond plate 380 b that correspond to the two sides of the first plate380 a are connected and that the respective remaining two sides are openallowing the transmission-type intermediate image screen 361 to be putin and out.

[Exemplary Variations of Screen Holding Unit]

In the above embodiment of the present invention, an explanation is maderegarding a case where the three-layer portion 380 is fixed while beingset in the storage space 392 of the screen holding unit 390. As long asthe screen holding unit 390 is able to fix the three-layer portion 380by physical force, the screen holding unit 390 is not required to beprovided with the storage space 392. For example, the screen holdingunit 390 may be shaped like a clear folder by forming the screen holdingunit 390 by two rectangular members that are transparent or that have anopening in order to allow image display light to be transmitted suchthat two adjacent sides are connected while the remaining two sides canbe freely opened and closed. In this case, the three-layer portion 380can be removed or installed by sliding the three-layer portion 380 intothe screen holding unit 390. Further, the screen holding unit 390, thefirst plate 380 a, and the second plate 380 b may be formed integrallyby forming the first plate 380 a and the second plate 380 b in anintegral manner on each of the above-described two rectangular members.

Also, the above explanation concerns a case where the screen holdingunit 390 is fixed while being set in the installation groove 310 in theprojection unit 300. However, any means may be used as long as thescreen holding unit 390 can be fixed to the projection unit 300, and theway of fixing the screen holding unit 390 is not limited to a case wherethe screen holding unit 390 is set in the installation groove 310. Forexample, the screen holding unit 390 and the projection unit 300 may befixed to each other using magnetic force. This can be achieved byplacing a magnet at the respective installation positions of the screenholding unit 390 and the projection unit 300.

Also, in the above-stated embodiment, an explanation is made for animage display device used as a display device for a vehicle that isplaced in a vehicle. Alternatively, the image display device may bethose used for other applications. For example, a lens surface ofglasses or the like worn by a user may be used as a combiner, and avirtual image may be presented to the user by projecting image displaylight onto the combiner.

What is claimed is:
 1. An image display device comprising: a projectionport that projects image display light generated based on an imagesignal; and a combiner that presents a virtual image by reflecting theimage display light projected from the projection port; wherein: withrespect to a specific direction along a reflection surface of thecombiner, the curvature of the combiner in the specific directionbecomes smaller as a distance from the projection port becomes larger;and the reflection surface of the combiner is formed by a biconicsurface, wherein, when a visual-line direction in which the virtualimage is visually recognized via the combiner is set to be a z-axisdirection, a direction that is perpendicular to the z-axis direction isset to be an x-axis direction, and a direction that is perpendicular tothe z-axis direction and the x-axis direction is set to be a y-axisdirection, the combiner is provided at a position where a projectionposition at which a projection axis of the image display light projectedfrom the projection port intersects with the reflection surface isdifferent from a vertex position at which both the curvature in thex-axis direction and the curvature in the y-axis direction of thebiconic surface forming the reflection surface become maximum, andwherein the combiner is shaped into a form such that a line ofintersection of the biconic surface forming the reflection surface andthe x-z plane is an ellipsoid line and such that a line of intersectionof the biconic surface forming the reflection surface and the y-z planeis a hyperbolic curve.
 2. The image display device according to claim 1wherein the projection position on the reflection surface and the vertexposition on the reflection surface are different from each other in they-axis direction in the combiner.
 3. The image display device accordingto claim 1, wherein the reflection surface of the combiner is formed bya shape that is asymmetrical using the lower half of a biconic surface.